Handover techniques in wireless communications

ABSTRACT

Methods, systems, and devices for wireless communications are described that provide a handover may be performed based on one or more conditions at a UE and in conjunction with a handover procedure from a source base station to a target base station. The one or more conditions at the UE may be associated with successful establishment or maintenance of the second connection. In some cases, the one or more conditions at the UE may correspond to one or more measurements associated with the source base station, the target base station, one or more neighboring base stations, or any combinations thereof.

BACKGROUND

The following relates generally to wireless communications, and morespecifically to handover techniques in wireless communications.

Wireless communications systems are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be capable ofsupporting communication with multiple users by sharing the availablesystem resources (e.g., time, frequency, and power). Examples of suchmultiple-access systems include fourth generation (4G) systems such asLong Term Evolution (LTE) systems, LTE-Advanced (LTE-A) systems, orLTE-A Pro systems, and fifth generation (5G) systems which may bereferred to as New Radio (NR) systems. These systems may employtechnologies such as code division multiple access (CDMA), time divisionmultiple access (TDMA), frequency division multiple access (FDMA),orthogonal frequency division multiple access (OFDMA), or discreteFourier transform spread orthogonal frequency division multiplexing(DFT-S-OFDM). A wireless multiple-access communications system mayinclude a number of base stations or network access nodes, eachsimultaneously supporting communication for multiple communicationdevices, which may be otherwise known as user equipment (UE).

When operating in a wireless communications system, a UE may movebetween coverage areas of multiple different base stations. In caseswhere radio signals of a neighboring base station, which may be referredto as a target base station, will provide an enhanced connection with aUE relative to a currently serving (or source) base station, the UE maybe handed over from the source base station to the target base station.Such techniques may be referred to as handover procedures or mobilityprocedures, and help to provide continuous connectivity to a UE as itmoves in a wireless communications system. In some systems, a UE mayrelease an active connection with the source base station and establisha new connection with the target base station in response to a handovercommunication from the source base station. Enhanced techniques forperforming handover may help to enhance the overall efficiency andreliability of a wireless communications system.

SUMMARY

The described techniques relate to improved methods, systems, devices,and apparatuses for performing handovers in wireless communications. Invarious aspects, the described techniques provide that a handover may beperformed based on one or more conditions at a UE in conjunction with ahandover procedure from a source base station to a target base station.In some cases, the one or more conditions at the UE may be associatedwith successful establishment or maintenance of the second connection.In some cases, the one or more conditions at the UE may correspond toone or more measurements associated with the source base station, thetarget base station, one or more neighboring base stations, or anycombinations thereof.

In some cases, for example, the UE may be unable to establish the secondconnection, or the second connection may be established and then failshortly afterward. In such cases, the UE may report a radio link failureto the source base station, and maintain the first connection. In somecases, the source base station may discontinue the handover and initiatea new handover procedure at the UE to a different neighboring basestation.

Additionally or alternatively, the UE may perform a measurement as partof the handover, and the handover may be modified based at least in parton the measurement. For example, the measurement may be performed afterestablishment of the second connection at the target base station, wherethe second connection is a primary connection and the first connectionis a secondary connection. In such cases, the source and target basestations may perform a role switch based on the measurement (e.g., whenthe measurement indicates the first connection has better channelconditions than the second connection) to change the first connection tobe the primary connection and the second connection to be the secondaryconnection. In some cases, two or more role switches may be performedbased on multiple measurements provided by the UE in accordance with aping-pong handling procedure, until one of the connections remains theprimary connection for a predetermined time period.

In some cases, the UE may perform the measurement as part of a randomaccess procedure with the target base station and may measure channelquality of one or more neighboring base stations. In such cases, the UEmay transmit the measurement to the source base station in the eventthat a neighboring base station has a channel quality that exceeds athreshold. The source base station, based at least in part on themeasurement, may cancel the handover to the second base station andinitiate a handover with the neighboring base station.

In some cases, the UE may transmit a measurement report to the sourcebase station that may initiate a handover of the UE from the source basestation to the target base station. In some cases, a second connectionwith the target base station may be established using a dualconnectivity (DC) technique, in which the first connection is changed toa secondary connection and the newly established second connection isset as a primary connection. In some cases, the source base station maymaintain the first connection as a primary connection, and switch thefirst connection to be a secondary connection responsive to a roleswitch communication with the target base station. In some cases, therole switch communication with the target base station may be combinedwith a handover request to the target base station and an associatedacknowledgment from the target base station.

A method of wireless communication is described. The method may includeestablishing, at a UE, a first connection with a first base station,receiving a handover message from the first base station to perform ahandover procedure with a second base station, transmitting, responsiveto the handover message, a request to the second base station toestablish a second connection with the second base station, where thefirst connection with the first base station is maintained during thehandover procedure, determining that a radio link failure of the firstconnection or the second connection has occurred, and transmitting anindication of the radio link failure to the first base station or thesecond base station responsive to the determining.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to establish, at aUE, a first connection with a first base station, receive a handovermessage from the first base station to perform a handover procedure witha second base station, transmit, responsive to the handover message, arequest to the second base station to establish a second connection withthe second base station, where the first connection with the first basestation is maintained during the handover procedure, determine that aradio link failure of the first connection or the second connection hasoccurred, and transmit an indication of the radio link failure to thefirst base station or the second base station responsive to thedetermining.

Another apparatus for wireless communication is described. The apparatusmay include means for establishing, at a UE, a first connection with afirst base station, receiving a handover message from the first basestation to perform a handover procedure with a second base station,transmitting, responsive to the handover message, a request to thesecond base station to establish a second connection with the secondbase station, where the first connection with the first base station ismaintained during the handover procedure, determining that a radio linkfailure of the first connection or the second connection has occurred,and transmitting an indication of the radio link failure to the firstbase station or the second base station responsive to the determining.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to establish, at a UE, a first connection with a firstbase station, receive a handover message from the first base station toperform a handover procedure with a second base station, transmit,responsive to the handover message, a request to the second base stationto establish a second connection with the second base station, where thefirst connection with the first base station is maintained during thehandover procedure, determine that a radio link failure of the firstconnection or the second connection has occurred, and transmit anindication of the radio link failure to the first base station or thesecond base station responsive to the determining.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the determining may includeoperations, features, means, or instructions for determining thatestablishment of the connection with the second base station may havefailed or the second connection failed after establishment of the secondconnection, and maintaining the first connection with the first basestation. In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the indication of the radiolink failure includes a failure cause associated with the secondconnection and a target cell group identification associated with thesecond base station. In some examples of the method, apparatuses, andnon-transitory computer-readable medium described herein, the indicationof the radio link failure further indicates one or more of a timerexpiration associated with the second connection, a random accessprocedure failure, or a maximum number of retransmissions is reached fora communication using the second connection. In some examples of themethod, apparatuses, and non-transitory computer-readable mediumdescribed herein, the indication of the radio link failure furtherindicates a measurement of one or more signals received at the UE fromone or more neighboring base stations.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the determining may includeoperations, features, means, or instructions for determining that thefirst connection with the first base station may have failed prior tocompletion of an establishment of the second connection, and completingthe establishment of the second connection with the second base station.In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the completing theestablishment of the second connection preempts a triggering of areestablishment of the first connection with the first base station.Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for releasing the firstconnection and a source stack associated with the first connectionresponsive to completing the establishment of the second connection withthe second base station. In some examples of the method, apparatuses,and non-transitory computer-readable medium described herein, theindication of the radio link failure indicates a failure causeassociated with the first connection and a source cell groupidentification associated with the first base station.

A method of wireless communication is described. The method may includeestablishing, at a first base station, a first connection with a UE,initiating a handover procedure to handover the UE to a second basestation, where the first connection with the first base station ismaintained during the handover procedure, receiving, from the UE, anindication of a failure of the handover procedure, and discontinuing thehandover procedure to handover the UE to the second base station.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to establish, at afirst base station, a first connection with a UE, initiate a handoverprocedure to handover the UE to a second base station, where the firstconnection with the first base station is maintained during the handoverprocedure, receive, from the UE, an indication of a failure of thehandover procedure, and discontinue the handover procedure to handoverthe UE to the second base station.

Another apparatus for wireless communication is described. The apparatusmay include means for establishing, at a first base station, a firstconnection with a UE, initiating a handover procedure to handover the UEto a second base station, where the first connection with the first basestation is maintained during the handover procedure, receiving, from theUE, an indication of a failure of the handover procedure, anddiscontinuing the handover procedure to handover the UE to the secondbase station.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to establish, at a first base station, a first connectionwith a UE, initiate a handover procedure to handover the UE to a secondbase station, where the first connection with the first base station ismaintained during the handover procedure, receive, from the UE, anindication of a failure of the handover procedure, and discontinue thehandover procedure to handover the UE to the second base station.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the receiving the indicationof the failure of the handover procedure further may include operations,features, means, or instructions for receiving a radio link failuremessage from the UE indicating a timer expiration associated with arandom access procedure to establish a second connection with the secondbase station. Some examples of the method, apparatuses, andnon-transitory computer-readable medium described herein may furtherinclude operations, features, means, or instructions for receiving aradio link failure message from the UE indicating a second connectionwith the second base station was established and then failed. In someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the radio link failuremessage indicates a failure cause associated with the second connection,and where the failure cause indicates one or more or a timer expirationassociated with the second connection, a random access procedurefailure, or a maximum number of retransmissions is reached for acommunication using the second connection. In some examples of themethod, apparatuses, and non-transitory computer-readable mediumdescribed herein, the radio link failure message further indicates atarget cell group identification associated with the second basestation. In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the indication of the failureof the handover procedure further indicates a measurement of one or moresignals received at the UE from a neighboring base station.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for transmitting, to thesecond base station and responsive to the receiving the indication ofthe failure of the handover procedure, a message to the second basestation to cancel the handover procedure.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for initiating a secondhandover procedure to handover the UE to a third base station, where thefirst connection with the first base station may be maintained duringthe second handover procedure. In some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein, the initiating the second handover procedure further may includeoperations, features, means, or instructions for transmitting, to theUE, a reconfiguration message indicating the UE is to perform the secondhandover procedure with the third base station.

A method of wireless communication is described. The method may includereceiving, at a second base station, a handover message to initiate ahandover of a UE from a first base station to the second base station,initiating, responsive to the handover message, a connectionestablishment with the UE to establish a second connection between theUE and the second base station, receiving from the UE an indication of afailure of a first connection between the UE and the first base station,where the first connection was to be maintained during the handover ofthe UE from the first base station to the second base station, andforwarding the indication of the failure of the first connection to thefirst base station.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to receive, at asecond base station, a handover message to initiate a handover of a UEfrom a first base station to the second base station, initiate,responsive to the handover message, a connection establishment with theUE to establish a second connection between the UE and the second basestation, receive from the UE an indication of a failure of a firstconnection between the UE and the first base station, where the firstconnection was to be maintained during the handover of the UE from thefirst base station to the second base station, and forward theindication of the failure of the first connection to the first basestation.

Another apparatus for wireless communication is described. The apparatusmay include means for receiving, at a second base station, a handovermessage to initiate a handover of a UE from a first base station to thesecond base station, initiating, responsive to the handover message, aconnection establishment with the UE to establish a second connectionbetween the UE and the second base station, receiving from the UE anindication of a failure of a first connection between the UE and thefirst base station, where the first connection was to be maintainedduring the handover of the UE from the first base station to the secondbase station, and forwarding the indication of the failure of the firstconnection to the first base station.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to receive, at a second base station, a handover messageto initiate a handover of a UE from a first base station to the secondbase station, initiate, responsive to the handover message, a connectionestablishment with the UE to establish a second connection between theUE and the second base station, receive from the UE an indication of afailure of a first connection between the UE and the first base station,where the first connection was to be maintained during the handover ofthe UE from the first base station to the second base station, andforward the indication of the failure of the first connection to thefirst base station.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the indication of the failureof the first connection indicates that resources and context associatedwith the first connection is to be deleted. In some examples of themethod, apparatuses, and non-transitory computer-readable mediumdescribed herein, the initiating the connection establishment with theUE further may include operations, features, means, or instructions forperforming a random access procedure with the UE to establish the secondconnection between the UE and the second base station, and where theindication of the failure of the first connection may be receivedsubsequent to the establishment of the second connection. In someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the indication of the failureof the first connection includes a failure cause associated with thefirst connection and a source cell group identification associated withthe first base station. In some examples of the method, apparatuses, andnon-transitory computer-readable medium described herein, the indicationof the failure of the first connection further includes a measurement ofone or more signals received at the UE from a neighboring base station.

A method of wireless communication is described. The method may includeestablishing, at a UE, a first connection with a first base station,receiving a handover message from the first base station to perform ahandover procedure with a second base station, initiating, at the UE, aconnection establishment with the second base station to establish asecond connection responsive to the handover message, where the firstconnection is maintained during the connection establishment with thesecond base station, measuring one or more channel conditions associatedwith one or more of the first base station, the second base station or athird base station, and transmitting, responsive to the handovermessage, a measurement report to at least one of the first base stationor the second base station responsive to measuring the one or morechannel conditions.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to establish, at aUE, a first connection with a first base station, receive a handovermessage from the first base station to perform a handover procedure witha second base station, initiate, at the UE, a connection establishmentwith the second base station to establish a second connection responsiveto the handover message, where the first connection is maintained duringthe connection establishment with the second base station, measure oneor more channel conditions associated with one or more of the first basestation, the second base station or a third base station, and transmit,responsive to the handover message, a measurement report to at least oneof the first base station or the second base station responsive tomeasuring the one or more channel conditions.

Another apparatus for wireless communication is described. The apparatusmay include means for establishing, at a UE, a first connection with afirst base station, receiving a handover message from the first basestation to perform a handover procedure with a second base station,initiating, at the UE, a connection establishment with the second basestation to establish a second connection responsive to the handovermessage, where the first connection is maintained during the connectionestablishment with the second base station, measuring one or morechannel conditions associated with one or more of the first basestation, the second base station or a third base station, andtransmitting, responsive to the handover message, a measurement reportto at least one of the first base station or the second base stationresponsive to measuring the one or more channel conditions.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to establish, at a UE, a first connection with a firstbase station, receive a handover message from the first base station toperform a handover procedure with a second base station, initiate, atthe UE, a connection establishment with the second base station toestablish a second connection responsive to the handover message, wherethe first connection is maintained during the connection establishmentwith the second base station, measure one or more channel conditionsassociated with one or more of the first base station, the second basestation or a third base station, and transmit, responsive to thehandover message, a measurement report to at least one of the first basestation or the second base station responsive to measuring the one ormore channel conditions.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for establishing the secondconnection with the second base station, where the second connection isa primary connection and the first connection is maintained as asecondary connection, receiving, responsive to the measurement report, areconfiguration message from the second base station to reconfigure thefirst connection as the primary connection and the second connection asthe secondary connection, and reconfiguring the first connection and thesecond connection responsive to the reconfiguration message.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for transmitting one ormore additional measurement reports via the primary connection. In someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the measurement report may betransmitted responsive to the measured one or more channel conditions ofthe secondary connection being better than corresponding channelconditions of the primary connection.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the reconfiguring the firstconnection and the second connection responsive to the reconfigurationmessage may be performed without performing a random access procedure.Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for transmitting andreceiving duplicated data via each of the primary connection and thesecondary connection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for releasing the firstconnection after the second connection may have been a primaryconnection for a predetermined time period. Some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein may further include operations, features, means, or instructionsfor receiving, responsive to the measurement report, a reconfigurationmessage from the first base station to abort the handover procedure withthe second base station. In some examples of the method, apparatuses,and non-transitory computer-readable medium described herein, themeasurement report may be transmitted responsive to measured channelconditions of the third base station being better than measured channelconditions of the second base station.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the reconfiguration messagefurther includes a second handover message from the first base stationto perform a second handover procedure with the third base station.

A method of wireless communication is described. The method may includeestablishing, at a first base station, a first connection with a UE,initiating a handover of the UE to a second base station, where thefirst connection with the first base station is maintained during thehandover and an establishment of a second connection between the UE andthe second base station, receiving a measurement report responsive tothe initiating the handover, and modifying the handover of the UE basedon the measurement report.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to establish, at afirst base station, a first connection with a UE, initiate a handover ofthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover and anestablishment of a second connection between the UE and the second basestation, receive a measurement report responsive to the initiating thehandover, and modify the handover of the UE based on the measurementreport.

Another apparatus for wireless communication is described. The apparatusmay include means for establishing, at a first base station, a firstconnection with a UE, initiating a handover of the UE to a second basestation, where the first connection with the first base station ismaintained during the handover and an establishment of a secondconnection between the UE and the second base station, receiving ameasurement report responsive to the initiating the handover, andmodifying the handover of the UE based on the measurement report.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to establish, at a first base station, a first connectionwith a UE, initiate a handover of the UE to a second base station, wherethe first connection with the first base station is maintained duringthe handover and an establishment of a second connection between the UEand the second base station, receive a measurement report responsive tothe initiating the handover, and modify the handover of the UE based onthe measurement report.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for transmitting a handoverindication to the second base station, receiving, from the second basestation prior to receiving the measurement report, a role switchindication that indicates that a second connection between the UE andthe second base station is a primary connection of the UE and the firstconnection is a secondary connection, and where the modifying thehandover of the UE includes reconfiguring the first connection to be theprimary connection and the second connection to be the secondaryconnection based on the measurement report.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving one or moreadditional measurement reports, and further reconfiguring the primaryconnection and the secondary connection based on the one or moreadditional measurement reports. In some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein, the measurement report may be transmitted responsive to themeasured one or more channel conditions of the secondary connectionbeing better than corresponding channel conditions of the primaryconnection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for exchanging informationwith the second base station to establish duplication of datacommunicated with the UE via both the first connection and the secondconnection, and communicating duplicated data with the UE via the firstconnection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for releasing the firstconnection after the second connection may have been a primaryconnection for a predetermined time period. In some examples of themethod, apparatuses, and non-transitory computer-readable mediumdescribed herein, the modifying the handover of the UE may includeoperations, features, means, or instructions for transmitting,responsive to the measurement report, a reconfiguration message to theUE to abort the handover procedure with the second base station. In someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the modifying the handover ofthe UE further may include operations, features, means, or instructionsfor transmitting an indication to the second base station that thehandover of the UE to the second base station may be aborted.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the measurement report may betransmitted responsive to measured channel conditions of a third basestation being better than measured channel conditions of the second basestation. In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the reconfiguration messagefurther includes a second handover message to the UE to perform a secondhandover procedure with a third base station.

A method of wireless communication is described. The method may includereceiving, at a second base station from a first base station, ahandover message to initiate a handover of a UE from the first basestation to the second base station, establishing a second connectionwith the UE responsive to the handover message, receiving a measurementreport from the UE that includes one or more channel measurementsassociated with the first base station and the second base station, andmodifying the handover of the UE based on the measurement report.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to receive, at asecond base station from a first base station, a handover message toinitiate a handover of a UE from the first base station to the secondbase station, establish a second connection with the UE responsive tothe handover message, receive a measurement report from the UE thatincludes one or more channel measurements associated with the first basestation and the second base station, and modify the handover of the UEbased on the measurement report.

Another apparatus for wireless communication is described. The apparatusmay include means for receiving, at a second base station from a firstbase station, a handover message to initiate a handover of a UE from thefirst base station to the second base station, establishing a secondconnection with the UE responsive to the handover message, receiving ameasurement report from the UE that includes one or more channelmeasurements associated with the first base station and the second basestation, and modifying the handover of the UE based on the measurementreport.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to receive, at a second base station from a first basestation, a handover message to initiate a handover of a UE from thefirst base station to the second base station, establish a secondconnection with the UE responsive to the handover message, receive ameasurement report from the UE that includes one or more channelmeasurements associated with the first base station and the second basestation, and modify the handover of the UE based on the measurementreport.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for transmitting, to thefirst base station prior to receiving the measurement report, a roleswitch indication that indicates that a second connection between the UEand the second base station is a primary connection of the UE and afirst connection between the IE and the first base station is asecondary connection, and where the modifying the handover of the UEincludes reconfiguring the first connection to be the primary connectionand the second connection to be the secondary connection based on themeasurement report.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving one or moreadditional measurement reports, and further reconfiguring the primaryconnection and the secondary connection based on the one or moreadditional measurement reports. In some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein, the measurement report may be transmitted by the UE responsiveto the measured one or more channel conditions of the secondaryconnection being better than corresponding channel conditions of theprimary connection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for exchanging informationwith the first base station to establish duplication of datacommunicated with the UE via both the first connection and the secondconnection, and communicating duplicated data with the UE via the secondconnection.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for releasing the secondconnection after the first connection has been a primary connection fora predetermined time period.

A method of wireless communication is described. The method may includeestablishing, at a first base station, a first connection with a UE,initiating a handover of the UE to a second base station, where thefirst connection with the first base station is maintained during thehandover and an establishment of a second connection between the UE andthe second base station, and receiving, from the second base station, arole switch indication that indicates that the second connection betweenthe UE and the second base station is a primary connection of the UE.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to establish, at afirst base station, a first connection with a UE, initiate a handover ofthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover and anestablishment of a second connection between the UE and the second basestation, and receive, from the second base station, a role switchindication that indicates that the second connection between the UE andthe second base station is a primary connection of the UE.

Another apparatus for wireless communication is described. The apparatusmay include means for establishing, at a first base station, a firstconnection with a UE, initiating a handover of the UE to a second basestation, where the first connection with the first base station ismaintained during the handover and an establishment of a secondconnection between the UE and the second base station, and receiving,from the second base station, a role switch indication that indicatesthat the second connection between the UE and the second base station isa primary connection of the UE.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to establish, at a first base station, a first connectionwith a UE, initiate a handover of the UE to a second base station, wherethe first connection with the first base station is maintained duringthe handover and an establishment of a second connection between the UEand the second base station, and receive, from the second base station,a role switch indication that indicates that the second connectionbetween the UE and the second base station is a primary connection ofthe UE.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the initiating the handovermay include operations, features, means, or instructions for configuringone or more secondary node (SN) terminated bearers at the second basestation for the second connection, and transmitting a reconfigurationmessage to the UE that indicates the handover to the second basestation.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the initiating the handoverfurther may include operations, features, means, or instructions fortransmitting a role switch request to the second base station, andreceiving an acknowledgment of the role switch request from the secondbase station. In some examples of the method, apparatuses, andnon-transitory computer-readable medium described herein, the roleswitch request may be transmitted with a SN addition request to thesecond base station, and the acknowledgment of the role switch requestmay be received with a SN addition acknowledgment from the second basestation. In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the reconfiguration messageto the UE indicates a secondary carrier group associated with the secondconnection and a source cell group identification associated with thefirst connection. In some examples of the method, apparatuses, andnon-transitory computer-readable medium described herein, the roleswitch indication may be received subsequent to performance of a randomaccess procedure between the second base station and the UE. Someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for releasing the firstconnection with the UE responsive to the role switch indication.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the releasing the firstconnection further may include operations, features, means, orinstructions for transmitting a secondary node (SN) status transfermessage to the second base station to indicate completion of the roleswitch. Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving dataassociated with the UE during the handover, and forwarding the receiveddata to the second base station.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for configuring the UE tomaintain the first connection during the handover and to use the firstconnection for transmitting and receiving data while the secondconnection is established.

A method of wireless communication is described. The method may includereceiving, at a second base station from a first base station, ahandover message to initiate a handover of a UE from the first basestation to the second base station, establishing a second connectionwith the UE responsive to the handover message, and transmitting,responsive to the establishing the second connection, a role switchindication to the first base station that indicates that the secondconnection between the UE and the second base station is a primaryconnection of the UE.

An apparatus for wireless communication is described. The apparatus mayinclude a processor, memory in electronic communication with theprocessor, and instructions stored in the memory. The instructions maybe executable by the processor to cause the apparatus to receive, at asecond base station from a first base station, a handover message toinitiate a handover of a UE from the first base station to the secondbase station, establish a second connection with the UE responsive tothe handover message, and transmit, responsive to the establishing thesecond connection, a role switch indication to the first base stationthat indicates that the second connection between the UE and the secondbase station is a primary connection of the UE.

Another apparatus for wireless communication is described. The apparatusmay include means for receiving, at a second base station from a firstbase station, a handover message to initiate a handover of a UE from thefirst base station to the second base station, establishing a secondconnection with the UE responsive to the handover message, andtransmitting, responsive to the establishing the second connection, arole switch indication to the first base station that indicates that thesecond connection between the UE and the second base station is aprimary connection of the UE.

A non-transitory computer-readable medium storing code for wirelesscommunication is described. The code may include instructions executableby a processor to receive, at a second base station from a first basestation, a handover message to initiate a handover of a UE from thefirst base station to the second base station, establish a secondconnection with the UE responsive to the handover message, and transmit,responsive to the establishing the second connection, a role switchindication to the first base station that indicates that the secondconnection between the UE and the second base station is a primaryconnection of the UE.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the receiving the handovermessage further may include operations, features, means, or instructionsfor receiving an indication from the first base station to configure oneor more secondary node (SN) terminated bearers at the second basestation for the second connection. In some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein, the receiving the handover message further may includeoperations, features, means, or instructions for receiving a role switchrequest from the first base station that indicates the second connectionwith the UE is to be the primary connection of the UE, and transmittingan acknowledgment of the role switch request to the first base station.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the role switch request maybe received with a SN addition request from the first base station, andthe acknowledgment of the role switch request may be transmitted with aSN addition acknowledgment to the first base station.

In some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein, the establishing a secondconnection with the UE may include operations, features, means, orinstructions for performing a random access procedure with the UE toestablish the second connection. In some examples of the method,apparatuses, and non-transitory computer-readable medium describedherein, the role switch indication may be transmitted after theperforming the random access procedure.

Some examples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving, from thefirst base station responsive to the role switch indication, a secondarynode (SN) status transfer message, and transmitting an indication to theUE to release a first connection with the first base station. Someexamples of the method, apparatuses, and non-transitorycomputer-readable medium described herein may further includeoperations, features, means, or instructions for receiving, from thefirst base station, data associated with the UE that is received at thefirst base station during the handover.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a system for wireless communicationsthat supports handover techniques in wireless communications inaccordance with aspects of the present disclosure.

FIG. 2 illustrates an example of a wireless communications system thatsupports handover techniques in wireless communications in accordancewith aspects of the present disclosure.

FIG. 3 illustrates an example of a process flow that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 4 illustrates an example of dual connectivity handover protocolstacks in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 5 illustrates an example of a process flow that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 6 illustrates an example of a process flow that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 7 illustrates an example of a process flow that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 8 illustrates an example of a process flow that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIGS. 9 and 10 show block diagrams of devices that support handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 11 shows a block diagram of a communications manager that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure.

FIG. 12 shows a diagram of a system including a device that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure.

FIGS. 13 and 14 show block diagrams of devices that support handovertechniques in wireless communications in accordance with aspects of thepresent disclosure.

FIG. 15 shows a block diagram of a communications manager that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure.

FIG. 16 shows a diagram of a system including a device that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure.

FIGS. 17 through 28 show flowcharts illustrating methods that supporthandover techniques in wireless communications in accordance withaspects of the present disclosure.

DETAILED DESCRIPTION

Various aspects of the present disclosure provide enhanced techniquesfor handover in a wireless communications system. In some cases, a userequipment (UE) may establish a first connection with a first basestation, which may be referred to as a source base station. The UE mayreceive a handover message, such as a radio resource control (RRC)reconfiguration message to establish a second connection with a secondbase station, which may be referred to as a target base station. In somecases, the handover of the UE from the source base station to the targetbase station may be triggered by a measurement report from the UE thatindicates channel conditions at the source base station and target basestation that meet handover criteria. In some cases, the UE may maintainthe first connection while the second connection is established,according to a dual-connectivity (DC) handover technique or amake-before-break handover technique. Such a DC or make-before-breakhandover technique may allow the UE to remain in a RRC connected statewith one base station during the handover procedure, and may support azero millisecond or almost zero millisecond handover interruption.

In various aspects of the disclosure, described techniques provide thata handover may be performed based on one or more conditions at a UE inconjunction with the handover procedure. In some cases, the one or moreconditions at the UE may be associated with successful establishment ormaintenance of the second connection, or one or more measurements madeat the UE in conjunction with the handover. In some cases, for example,the UE may be unable to establish the second connection, or the secondconnection may be established and then fail shortly afterward. In suchcases, the UE may report a radio link failure to the source basestation, and maintain the first connection. In some cases, the sourcebase station may discontinue the handover and initiate a new handoverprocedure at the UE to a different neighboring base station.

In some aspects of the disclosure, a UE may perform a measurement aspart of a handover, and the handover may be modified based at least inpart on the measurement. For example, the measurement may be performedafter establishment of the second connection at the target base station,where the second connection is a primary connection and the firstconnection is a secondary connection. In such cases, the source andtarget base stations may perform a role switch based on the measurement(e.g., when the measurement indicates the first connection has betterchannel conditions than the second connection) to change the firstconnection to be the primary connection and the second connection to bethe secondary connection. In some cases, two or more role switches maybe performed based on multiple measurements provided by the UE inaccordance with a ping-pong handling procedure, until one of theconnections remains the primary connection for a predetermined timeperiod.

In some cases, the UE may perform the measurement as part of a randomaccess procedure with the target base station and may measure channelquality of one or more neighboring base stations. In such cases, the UEmay transmit the measurement to the source base station in the eventthat a neighboring base station has a channel quality that exceeds athreshold. The source base station, based at least in part on themeasurement, may cancel the handover to the second base station andinitiate a handover with the neighboring base station.

In some cases, a second connection with the target base station may beestablished using a dual connectivity (DC) technique, in which the firstconnection is changed to a secondary connection and the newlyestablished second connection is set as a primary connection. In suchcases, the source base station may switch the first connection to be asecondary connection responsive to a role switch communication with thetarget base station. In some cases, a role switch request may becombined with a handover request to the target base station and anassociated acknowledgment from the target base station may include arole switch request acknowledgment, and the target base station maytransmit a role switch setup complete upon establishment of the secondconnection that triggers the switch of the first connection to be asecondary connection.

Such techniques may provide for enhanced reliability and efficiency inhandovers, and may provide 0 ms or almost 0 ms interruption handovers.Such techniques may allow for a UE to remain in an RRC connected stateand allow to UE to transmit or receive communications during a handoverprocedure. Such connectivity may allow for enhanced communications thatmay have relatively strict latency and/or reliability requirements(e.g., ultra-reliable low latency communications (URLLC)) during UEmobility. Further, in some cases, a primary and secondary connection atthe UE may be switched on one or more occasions during a handoverprocedure, which may allow communications on a relatively reliableconnection, which may enhance overall reliability of communications.Additionally, in some cases a UE may measure one or more channelconditions that may allow for relatively fast handover to a morepreferable base station, and thus further enhance network reliability.

Aspects of the disclosure are initially described in the context of awireless communications system. Handover techniques in accordance withvarious examples and then discussed. Aspects of the disclosure arefurther illustrated by and described with reference to apparatusdiagrams, system diagrams, and flowcharts that relate to handovertechniques in wireless communications.

FIG. 1 illustrates an example of a wireless communications system 100that supports handover techniques in wireless communications inaccordance with aspects of the present disclosure. The wirelesscommunications system 100 includes base stations 105, UEs 115, and acore network 130. In some examples, the wireless communications system100 may be a Long Term Evolution (LTE) network, an LTE-Advanced (LTE-A)network, an LTE-A Pro network, or a New Radio (NR) network. In somecases, wireless communications system 100 may support enhanced broadbandcommunications, ultra-reliable (e.g., mission critical) communications,low latency communications, or communications with low-cost andlow-complexity devices. UEs 115 may move within the wirelesscommunications system, and may perform handovers between different basestations 105 in accordance with one or more handover techniques asdiscussed herein.

Base stations 105 may wirelessly communicate with UEs 115 via one ormore base station antennas. Base stations 105 described herein mayinclude or may be referred to by those skilled in the art as a basetransceiver station, a radio base station, an access point, a radiotransceiver, a NodeB, an eNodeB (eNB), a next-generation NodeB orgiga-NodeB (either of which may be referred to as a gNB), a Home NodeB,a Home eNodeB, or some other suitable terminology. Wirelesscommunications system 100 may include base stations 105 of differenttypes (e.g., macro or small cell base stations). The UEs 115 describedherein may be able to communicate with various types of base stations105 and network equipment including macro eNBs, small cell eNBs, gNBs,relay base stations, and the like.

Each base station 105 may be associated with a particular geographiccoverage area 110 in which communications with various UEs 115 issupported. Each base station 105 may provide communication coverage fora respective geographic coverage area 110 via communication links 125,and communication links 125 between a base station 105 and a UE 115 mayutilize one or more carriers. Communication links 125 shown in wirelesscommunications system 100 may include uplink transmissions from a UE 115to a base station 105, or downlink transmissions from a base station 105to a UE 115. Downlink transmissions may also be called forward linktransmissions while uplink transmissions may also be called reverse linktransmissions.

The geographic coverage area 110 for a base station 105 may be dividedinto sectors making up only a portion of the geographic coverage area110, and each sector may be associated with a cell. For example, eachbase station 105 may provide communication coverage for a macro cell, asmall cell, a hot spot, or other types of cells, or various combinationsthereof. In some examples, a base station 105 may be movable andtherefore provide communication coverage for a moving geographiccoverage area 110. In some examples, different geographic coverage areas110 associated with different technologies may overlap, and overlappinggeographic coverage areas 110 associated with different technologies maybe supported by the same base station 105 or by different base stations105. The wireless communications system 100 may include, for example, aheterogeneous LTE/LTE-A/LTE-A Pro or NR network in which different typesof base stations 105 provide coverage for various geographic coverageareas 110.

The term “cell” refers to a logical communication entity used forcommunication with a base station 105 (e.g., over a carrier), and may beassociated with an identifier for distinguishing neighboring cells(e.g., a physical cell identifier (PCID), a virtual cell identifier(VCID)) operating via the same or a different carrier. In some examples,a carrier may support multiple cells, and different cells may beconfigured according to different protocol types (e.g., machine-typecommunication (MTC), narrowband Internet-of-Things (NB-IoT), enhancedmobile broadband (eMBB), or others) that may provide access fordifferent types of devices. In some cases, the term “cell” may refer toa portion of a geographic coverage area 110 (e.g., a sector) over whichthe logical entity operates, or may refer to a radio head or distributedunit (DU) and a base station 105 may control one or more cells.

UEs 115 may be dispersed throughout the wireless communications system100, and each UE 115 may be stationary or mobile. A UE 115 may also bereferred to as a mobile device, a wireless device, a remote device, ahandheld device, or a subscriber device, or some other suitableterminology, where the “device” may also be referred to as a unit, astation, a terminal, or a client. A UE 115 may also be a personalelectronic device such as a cellular phone, a personal digital assistant(PDA), a tablet computer, a laptop computer, or a personal computer. Insome examples, a UE 115 may also refer to a wireless local loop (WLL)station, an Internet of Things (IoT) device, an Internet of Everything(IoE) device, or an MTC device, or the like, which may be implemented invarious articles such as appliances, vehicles, meters, or the like.

Some UEs 115, such as MTC or IoT devices, may be low cost or lowcomplexity devices, and may provide for automated communication betweenmachines (e.g., via Machine-to-Machine (M2M) communication). M2Mcommunication or MTC may refer to data communication technologies thatallow devices to communicate with one another or a base station 105without human intervention. In some examples, M2M communication or MTCmay include communications from devices that integrate sensors or metersto measure or capture information and relay that information to acentral server or application program that can make use of theinformation or present the information to humans interacting with theprogram or application. Some UEs 115 may be designed to collectinformation or enable automated behavior of machines. Examples ofapplications for MTC devices include smart metering, inventorymonitoring, water level monitoring, equipment monitoring, healthcaremonitoring, wildlife monitoring, weather and geological eventmonitoring, fleet management and tracking, remote security sensing,physical access control, and transaction-based business charging.

In some cases, a UE 115 may also be able to communicate directly withother UEs 115 (e.g., using a peer-to-peer (P2P) or device-to-device(D2D) protocol). One or more of a group of UEs 115 utilizing D2Dcommunications may be within the geographic coverage area 110 of a basestation 105. Other UEs 115 in such a group may be outside the geographiccoverage area 110 of a base station 105, or be otherwise unable toreceive transmissions from a base station 105. In some cases, groups ofUEs 115 communicating via D2D communications may utilize a one-to-many(1:M) system in which each UE 115 transmits to every other UE 115 in thegroup. In some cases, a base station 105 facilitates the scheduling ofresources for D2D communications. In other cases, D2D communications arecarried out between UEs 115 without the involvement of a base station105.

Base stations 105 may communicate with the core network 130 and with oneanother. For example, base stations 105 may interface with the corenetwork 130 through backhaul links 132 (e.g., via an S1, N2, N3, orother interface). Base stations 105 may communicate with one anotherover backhaul links 134 (e.g., via an X2, Xn, or other interface) eitherdirectly (e.g., directly between base stations 105) or indirectly (e.g.,via core network 130).

The core network 130 may provide user authentication, accessauthorization, tracking, Internet Protocol (IP) connectivity, and otheraccess, routing, or mobility functions. The core network 130 may be anevolved packet core (EPC), which may include at least one mobilitymanagement entity (MME), at least one serving gateway (S-GW), and atleast one Packet Data Network (PDN) gateway (P-GW). The MME may managenon-access stratum (e.g., control plane) functions such as mobility,authentication, and bearer management for UEs 115 served by basestations 105 associated with the EPC. User IP packets may be transferredthrough the S-GW, which itself may be connected to the P-GW. The P-GWmay provide IP address allocation as well as other functions. The P-GWmay be connected to the network operators IP services. The operators IPservices may include access to the Internet, Intranet(s), an IPMultimedia Subsystem (IMS), or a Packet-Switched (PS) Streaming Service.

At least some of the network devices, such as a base station 105, mayinclude subcomponents such as an access network entity, which may be anexample of an access node controller (ANC) or centralized unit (CU).Each access network entity may communicate with UEs 115 through a numberof other access network transmission entities, which may be referred toas a radio head, a smart radio head, a transmission/reception point(TRP), or a distributed unit (DU). In some configurations, variousfunctions of each access network entity or base station 105 may bedistributed across various network devices (e.g., radio heads and accessnetwork controllers) or consolidated into a single network device (e.g.,a base station 105). For example, a CU may control two or more DUs,which may each be associated with a different cell.

Wireless communications system 100 may operate using one or morefrequency bands, typically in the range of 300 megahertz (MHz) to 300gigahertz (GHz). Generally, the region from 300 MHz to 3 GHz is known asthe ultra-high frequency (UHF) region or decimeter band, since thewavelengths range from approximately one decimeter to one meter inlength. UHF waves may be blocked or redirected by buildings andenvironmental features. However, the waves may penetrate structuressufficiently for a macro cell to provide service to UEs 115 locatedindoors. Transmission of UHF waves may be associated with smallerantennas and shorter range (e.g., less than 100 km) compared totransmission using the smaller frequencies and longer waves of the highfrequency (HF) or very high frequency (VHF) portion of the spectrumbelow 300 MHz.

Wireless communications system 100 may also operate in a super highfrequency (SHF) region using frequency bands from 3 GHz to 30 GHz, alsoknown as the centimeter band. The SHF region includes bands such as the5 GHz industrial, scientific, and medical (ISM) bands, which may be usedopportunistically by devices that may be capable of toleratinginterference from other users.

Wireless communications system 100 may also operate in an extremely highfrequency (EHF) region of the spectrum (e.g., from 30 GHz to 300 GHz),also known as the millimeter band. In some examples, wirelesscommunications system 100 may support millimeter wave (mmW)communications between UEs 115 and base stations 105, and EHF antennasof the respective devices may be even smaller and more closely spacedthan UHF antennas. In some cases, this may facilitate use of antennaarrays within a UE 115. However, the propagation of EHF transmissionsmay be subject to even greater atmospheric attenuation and shorter rangethan SHF or UHF transmissions. Techniques disclosed herein may beemployed across transmissions that use one or more different frequencyregions, and designated use of bands across these frequency regions maydiffer by country or regulating body.

In some cases, wireless communications system 100 may utilize bothlicensed and unlicensed radio frequency spectrum bands. For example,wireless communications system 100 may employ License Assisted Access(LAA), LTE-Unlicensed (LTE-U) radio access technology, or NR technologyin an unlicensed band such as the 5 GHz ISM band. When operating inunlicensed radio frequency spectrum bands, wireless devices such as basestations 105 and UEs 115 may employ listen-before-talk (LBT) proceduresto ensure a frequency channel is clear before transmitting data. In somecases, operations in unlicensed bands may be based on a carrieraggregation configuration in conjunction with component carriersoperating in a licensed band (e.g., LAA). Operations in unlicensedspectrum may include downlink transmissions, uplink transmissions,peer-to-peer transmissions, or a combination of these. Duplexing inunlicensed spectrum may be based on frequency division duplexing (FDD),time division duplexing (TDD), or a combination of both.

In some examples, base station 105 or UE 115 may be equipped withmultiple antennas, which may be used to employ techniques such astransmit diversity, receive diversity, multiple-input multiple-output(MIMO) communications, or beamforming. For example, wirelesscommunications system 100 may use a transmission scheme between atransmitting device (e.g., a base station 105) and a receiving device(e.g., a UE 115), where the transmitting device is equipped withmultiple antennas and the receiving device is equipped with one or moreantennas. MIMO communications may employ multipath signal propagation toincrease the spectral efficiency by transmitting or receiving multiplesignals via different spatial layers, which may be referred to asspatial multiplexing. The multiple signals may, for example, betransmitted by the transmitting device via different antennas ordifferent combinations of antennas. Likewise, the multiple signals maybe received by the receiving device via different antennas or differentcombinations of antennas. Each of the multiple signals may be referredto as a separate spatial stream, and may carry bits associated with thesame data stream (e.g., the same codeword) or different data streams.Different spatial layers may be associated with different antenna portsused for channel measurement and reporting. MIMO techniques includesingle-user MIMO (SU-MIMO) where multiple spatial layers are transmittedto the same receiving device, and multiple-user MIMO (MU-MIMO) wheremultiple spatial layers are transmitted to multiple devices.

Beamforming, which may also be referred to as spatial filtering,directional transmission, or directional reception, is a signalprocessing technique that may be used at a transmitting device or areceiving device (e.g., a base station 105 or a UE 115) to shape orsteer an antenna beam (e.g., a transmit beam or receive beam) along aspatial path between the transmitting device and the receiving device.Beamforming may be achieved by combining the signals communicated viaantenna elements of an antenna array such that signals propagating atparticular orientations with respect to an antenna array experienceconstructive interference while others experience destructiveinterference. The adjustment of signals communicated via the antennaelements may include a transmitting device or a receiving deviceapplying certain amplitude and phase offsets to signals carried via eachof the antenna elements associated with the device. The adjustmentsassociated with each of the antenna elements may be defined by abeamforming weight set associated with a particular orientation (e.g.,with respect to the antenna array of the transmitting device orreceiving device, or with respect to some other orientation).

In one example, a base station 105 may use multiple antennas or antennaarrays to conduct beamforming operations for directional communicationswith a UE 115. For instance, some signals (e.g., synchronizationsignals, reference signals, beam selection signals, or other controlsignals) may be transmitted by a base station 105 multiple times indifferent directions, which may include a signal being transmittedaccording to different beamforming weight sets associated with differentdirections of transmission. Transmissions in different beam directionsmay be used to identify (e.g., by the base station 105 or a receivingdevice, such as a UE 115) a beam direction for subsequent transmissionand/or reception by the base station 105.

Some signals, such as data signals associated with a particularreceiving device, may be transmitted by a base station 105 in a singlebeam direction (e.g., a direction associated with the receiving device,such as a UE 115). In some examples, the beam direction associated withtransmissions along a single beam direction may be determined based atleast in in part on a signal that was transmitted in different beamdirections. For example, a UE 115 may receive one or more of the signalstransmitted by the base station 105 in different directions, and the UE115 may report to the base station 105 an indication of the signal itreceived with a highest signal quality, or an otherwise acceptablesignal quality. Although these techniques are described with referenceto signals transmitted in one or more directions by a base station 105,a UE 115 may employ similar techniques for transmitting signals multipletimes in different directions (e.g., for identifying a beam directionfor subsequent transmission or reception by the UE 115), or transmittinga signal in a single direction (e.g., for transmitting data to areceiving device).

A receiving device (e.g., a UE 115, which may be an example of a mmWreceiving device) may try multiple receive beams when receiving varioussignals from the base station 105, such as synchronization signals,reference signals, beam selection signals, or other control signals. Forexample, a receiving device may try multiple receive directions byreceiving via different antenna subarrays, by processing receivedsignals according to different antenna subarrays, by receiving accordingto different receive beamforming weight sets applied to signals receivedat a plurality of antenna elements of an antenna array, or by processingreceived signals according to different receive beamforming weight setsapplied to signals received at a plurality of antenna elements of anantenna array, any of which may be referred to as “listening” accordingto different receive beams or receive directions. In some examples areceiving device may use a single receive beam to receive along a singlebeam direction (e.g., when receiving a data signal). The single receivebeam may be aligned in a beam direction determined based at least inpart on listening according to different receive beam directions (e.g.,a beam direction determined to have a highest signal strength, highestsignal-to-noise ratio, or otherwise acceptable signal quality based atleast in part on listening according to multiple beam directions).

In some cases, the antennas of a base station 105 or UE 115 may belocated within one or more antenna arrays, which may support MIMOoperations, or transmit or receive beamforming. For example, one or morebase station antennas or antenna arrays may be co-located at an antennaassembly, such as an antenna tower. In some cases, antennas or antennaarrays associated with a base station 105 may be located in diversegeographic locations. A base station 105 may have an antenna array witha number of rows and columns of antenna ports that the base station 105may use to support beamforming of communications with a UE 115.Likewise, a UE 115 may have one or more antenna arrays that may supportvarious MIMO or beamforming operations.

In some cases, wireless communications system 100 may be a packet-basednetwork that operate according to a layered protocol stack. In the userplane, communications at the bearer or Packet Data Convergence Protocol(PDCP) layer may be IP-based. A Radio Link Control (RLC) layer mayperform packet segmentation and reassembly to communicate over logicalchannels. A Medium Access Control (MAC) layer may perform priorityhandling and multiplexing of logical channels into transport channels.The MAC layer may also use hybrid automatic repeat request (HARQ) toprovide retransmission at the MAC layer to improve link efficiency. Inthe control plane, the Radio Resource Control (RRC) protocol layer mayprovide establishment, configuration, and maintenance of an RRCconnection between a UE 115 and a base station 105 or core network 130supporting radio bearers for user plane data. At the Physical layer,transport channels may be mapped to physical channels.

In some cases, UEs 115 and base stations 105 may support retransmissionsof data to increase the likelihood that data is received successfully.HARQ feedback is one technique of increasing the likelihood that data isreceived correctly over a communication link 125. HARQ may include acombination of error detection (e.g., using a cyclic redundancy check(CRC)), forward error correction (FEC), and retransmission (e.g.,automatic repeat request (ARQ)). HARQ may improve throughput at the MAClayer in poor radio conditions (e.g., signal-to-noise conditions). Insome cases, a wireless device may support same-slot HARQ feedback, wherethe device may provide HARQ feedback in a specific slot for datareceived in a previous symbol in the slot. In other cases, the devicemay provide HARQ feedback in a subsequent slot, or according to someother time interval.

The term “carrier” refers to a set of radio frequency spectrum resourceshaving a defined physical layer structure for supporting communicationsover a communication link 125. For example, a carrier of a communicationlink 125 may include a portion of a radio frequency spectrum band thatis operated according to physical layer channels for a given radioaccess technology. Each physical layer channel may carry user data,control information, or other signaling. A carrier may be associatedwith a pre-defined frequency channel (e.g., an evolved universal mobiletelecommunication system terrestrial radio access (E-UTRA) absoluteradio frequency channel number (EARFCN)), and may be positionedaccording to a channel raster for discovery by UEs 115. Carriers may bedownlink or uplink (e.g., in an FDD mode), or be configured to carrydownlink and uplink communications (e.g., in a TDD mode). In someexamples, signal waveforms transmitted over a carrier may be made up ofmultiple sub-carriers (e.g., using multi-carrier modulation (MCM)techniques such as orthogonal frequency division multiplexing (OFDM) ordiscrete Fourier transform spread OFDM (DFT-S-OFDM)).

The organizational structure of the carriers may be different fordifferent radio access technologies (e.g., LTE, LTE-A, LTE-A Pro, NR).For example, communications over a carrier may be organized according toTTIs or slots, each of which may include user data as well as controlinformation or signaling to support decoding the user data. A carriermay also include dedicated acquisition signaling (e.g., synchronizationsignals or system information, etc.) and control signaling thatcoordinates operation for the carrier. In some examples (e.g., in acarrier aggregation configuration), a carrier may also have acquisitionsignaling or control signaling that coordinates operations for othercarriers.

Physical channels may be multiplexed on a carrier according to varioustechniques. A physical control channel and a physical data channel maybe multiplexed on a downlink carrier, for example, using time divisionmultiplexing (TDM) techniques, frequency division multiplexing (FDM)techniques, or hybrid TDM-FDM techniques. In some examples, controlinformation transmitted in a physical control channel may be distributedbetween different control regions in a cascaded manner (e.g., between acommon control region or common search space and one or more UE-specificcontrol regions or UE-specific search spaces).

Wireless communications system 100 may support communication with a UE115 on multiple cells or carriers, a feature which may be referred to ascarrier aggregation or multi-carrier operation. A UE 115 may beconfigured with multiple downlink component carriers and one or moreuplink component carriers according to a carrier aggregationconfiguration. Carrier aggregation may be used with both FDD and TDDcomponent carriers.

In some cases, a UE 115 may perform a handover procedure from a sourcebase station 105 to a target base station 105 when moving within thewireless communications system. In some cases, such a handover may beperformed in accordance with one or more handover techniques providedherein that may provide a 0 ms or almost 0 ms interruption handover atthe UE 115.

FIG. 2 illustrates an example of a wireless communications system 200that supports handover techniques in wireless communications inaccordance with aspects of the present disclosure. In some examples,wireless communications system 200 may implement aspects of wirelesscommunications system 100. In some examples, wireless communicationssystem 200 may implement aspects of wireless communications system 100.The wireless communications system 200 may include a first base station105-a, a second base station 105-b, and UE 115-a, which may be examplesof a base station 105 and a UE 115, as described with reference to FIG.1.

First base station 105-a may be a source base station 105 and the secondbase station 105-b may be a target base station 105 in a handover 215 ofthe UE 115-a from the first base station 105-a to the second basestation 105-b. First base station 105-a and second base station 105-bmay be in communication with each other, such as via backhaul link 134-a(e.g., via an X2, Xn, or other interface), which may be a wired orwireless interface. While the example of FIG. 2 shows the first basestation 105-a in direct communication with the second base station105-b, in other cases the communication may be indirect, such as via acore network (e.g., core network 130 or FIG. 1). In this example, the UE115-a and the first base station 105-a may establish a first connection205. In the event that a handover is triggered, the UE 115-a mayestablish a second connection 210 with the second base station 105-b.Various techniques as discussed herein provide for efficient handoversthat may provide zero ms or almost 0 ms interruptions in the handover215 of the UE 115-a.

In some traditional systems, when performing a handover 215 from thefirst base station 105-a to the second base station 105-b, the UE 115-amay release the first connection 205 and perform a connectionestablishment procedure (e.g., a random access procedure) to establishthe second connection 210. In such cases, in the event that data is tobe communicated with the UE 115-a, there may be an interruption inconnectivity with the UE 115-a after the first connection 205 isreleased and until the second connection 210 is established. Whenperforming a 0 ms or almost 0 ms handover, such an interruption iseliminated or almost eliminated, and thus connectivity with the UE 115-ais enhanced.

In some cases, the handover 215 of the UE 115-a from the first basestation 105-a to the second base station 105-b may be amake-before-break handover in which the UE 115-a may have a capabilityto simultaneously transmit and receive communications, and the UE 115-amay maintain the first connection 205 while the second connection 210 isbeing established. In other cases, the handover 215 of the UE 115-a maybe a dual-connectivity (DC) handover, in which the UE 115-a has acapability of maintaining multiple connections with multiple cells andthe second connection 210 is established according to a DC procedurebefore the first connection 205 is released. In some cases, a servingbase station 105 (e.g., first base station 105-a) may identify acapability of the UE 115-a for maintaining multiple connections during ahandover, such as, for example, based on a capability indicationtransmitted by the UE 115-a, or a UE category indicated by the UE 115-a,during a connection establishment procedure to establish the firstconnection 205. Based on the UE 115-a capability, the first base station105-a may configure the UE 115-a to perform a make-before-break handoveror a DC handover (e.g., via radio resource control (RRC) signaling).

In some cases, the first base station 105-a may determine that the UE115-a is to be handed over to the second base station 105-b (e.g., basedon a measurement report provided by the UE 115-a). Based on such adetermination, the first base station 105-a may transmit a RRCreconfiguration to the UE 115-a that may indicate that UE 115-a is toperform a make-before-break handover procedure (or DC handoverprocedure) with the second base station 105-b using a simultaneoustransmit and receive capability of UE 115-a. In such a case, UE 115-amay determine that the first connection 205 is to be maintained with thefirst base station 105-a while establishing the second connection 210with the second base station 105-b. For example, UE 115-a may perform arandom access procedure with the second base station 105-b (e.g., toinitiate and establish a connection with the second base station 105-b).In some aspects, UE 115-a can continue to exchange user data (e.g.,uplink/downlink user data) via the first base station 105-a during andafter the random access procedure. In some cases, after establishment ofthe second connection 210, the first connection 205 may be released.Various techniques for such handovers that provide for enhancedconnectivity and efficiency are described herein.

FIG. 3 illustrates an example of a process flow 300 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The process flow 300 may include afirst base station 105-c, a second base station 105-d, and a UE 115-b,which may be examples of the corresponding devices described withreference to FIGS. 1 and 2. In some examples, the process flow 300 mayimplement aspects of the wireless communications system 100 and 200. Forexample, the first base station 105-c, the second base station 105-d,and the UE 115-b, may support 0 ms or almost 0 ms interruptionhandovers.

In the following description of the process flow 300, the operationsbetween the first base station 105-c, second base station 105-d, and theUE 115-b may be transmitted in a different order than the exemplaryorder shown, or the operations performed by the base stations 105 andthe UE 115-b may be performed in different orders or at different times.Certain operations may also be left out of the process flow 300, orother operations may be added to the process flow 300.

In this example a handover between the first base station 105-c and thesecond base station 105-d (which may be referred to as a source and atarget, respectively) is an inter-CU handover, using a DC handover, inwhich both the first base station 105-c and the second base station105-d are associated with a different CU.

In some examples, the process flow 300 may commence at 305 with thefirst base station 105-c establishing a first connection with the UE115-b (e.g., performing a cell acquisition procedure, a random accessprocedure, an RRC connection procedure, an RRC configuration procedure,etc.).

At 310, an event trigger may occur that may cause the UE 115-b toperform a measurement procedure. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-b to identify one or more neighboring base stations thatmay be candidates for the handover procedure. In some cases, the eventtrigger may be a signal measurement (e.g., a reference signal receivedpower (RSRP)) of the first base station 105-c dropping below a thresholdvalue, may be a time-based periodic event, or combinations thereof. At315, the UE 115-c may transmit measurement report to the first basestation 105-c, and the measurement report may indicate that the secondbase station 105-d is a candidate base station for a handover.

At 320, the first base station may make a handover decision to handoverthe UE 115-b from the first base station 105-c to the second basestation 105-d. In some cases, the first base station 105-c may make thehandover decision based on the measurement report provided by the UE115-b, one or more threshold values for initiating a handover, adifferential in one or more signal measurements from one or more priormeasurement reports, one or more measurement reports from other UEs, orany combinations thereof. In the example of FIG. 3, the first basestation 105-c may determine that the UE 115-b is to be handed over tothe second base station 105-d. Further, the first base station 105-c maydetermine that the UE 115-b has a DC capability and may make a decisionto perform a DC based handover of the UE 115-b. For example, the firstbase station 105-c (or an associated CU) may select from possiblehandover procedures that may be performed by UE 115-b based at least inpart on the indicated capability of UE 115-b. In the example of FIG. 3,the first base station 105-c may select an DC handover procedure for UE115-b based at least in part on UE 115-b indicating a DC capability. Inother examples, the first base station 105-c may select amake-before-break handover procedure or a different handover procedure(e.g., a legacy handover procedure), and different operations associatedwith the selected handover procedure may be performed.

At 325, the first base station 105-c may transmit a secondary node (SN)addition request to the second base station 105-d. The SN additionrequest may be provided based on the DC handover decision, in order toestablish a DC connection between both the first base station 105-c andthe second base station 105-d and the UE 115-b. The initial connectionbetween the first base station 105-c and the UE 115-b in such cases is amaster node (MN) connection and the second base station 105-d may beadded initially as a SN based on the SN addition request. At 330, thesecond base station 105-d may transmit a SN addition acknowledgment tothe first base station 105-c to acknowledge that the second base station105-d received, and that may indicate an ability to serve UE 115-b afterthe handover procedure.

In some examples, at 335, the first base station 105-c may transmit arole switch request to the second base station 105-d. The role switchrequest may indicate that the second base station 105-d is to switch tobe the master node in the DC connection with the UE 115-b once theconnection between the UE 115-b and the second base station isestablished, and the first base station 105-c is to switch to be the SNconnection. The second base station 105-d may receive the role switchrequest and identify that the UE 115-b is to be handed over to thesecond base station 105-d, and transmit a role switch acknowledgment at340.

In some cases, the role switch request may be combined with the SNaddition request, and the role switch acknowledgment may be combinedwith the SN addition acknowledgment. In such cases, signaling betweenthe first base station 105-c and the second base station 105-d may bereduced. Thus, in the example of FIG. 3, the role switch request at 335and the role switch acknowledgment at 340 are shown as optionaloperations, and may be skipped in cases where the SN addition request at325 also includes the role switch request, and the SN additionacknowledgment at 330 also includes the role switch acknowledgment.

At 345, the first base station 105-c may transmit a RRC reconfigurationmessage to the UE 115-b. The RRC reconfiguration message may indicate tothe UE 115-b that a connection with the second base station 105-d is tobe established according to a DC connection establishment procedure. Forexample, the RRC reconfiguration may indicate that UE 115-b is toperform DC handover procedure with second base station 105-d using theDC capability of UE 115-b. In some aspects, the RRC reconfiguration caninclude information identifying second base station 105-d, informationidentifying a handover configuration, identifying a DC configuration, orcombinations thereof. For example, in some cases, the RRCreconfiguration message may provide an indication of a secondary cellgroup associated with the second base station 105-d that is to beconfigured as part of the DC based handover, and also indicate thesource cell group identification of the first base station 105-c. Insome cases, the operations at 310 through 345 may be referred to as aphase-I handover preparation portion of a handover.

Following the transmission of the RRC reconfiguration at 345 and until asecond connection with the second base station 105-d is established,downlink data that is to be transmitted to the UE 115-b may be providedto the first base station 105-c (e.g., from a user plane function (UPF)at the core network), and the first base station 105-c may transmit thedownlink data to the UE 115-b via the first connection. Similarly,uplink data transmitted from the UE 115-b may be transmitted to thefirst base station 105-c via the first connection. In some cases, thefirst base station 105-c may forward downlink user data to the secondbase station 105-d.

In some cases, at 346, the UE 115-b may configure a SN based on the RRCreconfiguration based on a DC connection procedure, and may transmit anRRC reconfiguration complete indication to the first base station 105-cto indicate that the second connection is being established. During theestablishment of the second connection with the second base station105-d, the UE 115-b may maintain the first connection with the firstbase station 105-c, and thus have an active established connectionduring the handover, as indicated at 397. As part of the DC connectionprocedure, at 347, the first base station 105-c may transmit a SNreconfiguration complete indication to the second base station 105-dthat may indicate that the UE 115-b is configuring the secondconnection.

At 350, the UE 115-b and the second base station 105-d may perform arandom access procedure to establish the second connection with thesecond base station 105-d. The random access procedure may be performedaccording to established random access procedures (e.g., random accesschannel (RACH) procedures that are established in LTE or NR). Uponcompletion of the random access procedure and establishment of thesecond connection, the UE 115-b may, at 355, transmit a RRCreconfiguration complete indication to the second base station 105-d. Atthis point, the second base station 105-d may be the master node in theDC connection with both the first base station 105-c and second basestation 105-d, and thus the second connection is the primary link asindicated at 360. In some cases, the UE 115-b may transmit a PDCP statustransfer and uplink user data to the second base station 105-d via thesecond connection. In some aspects, after the RRC reconfiguration iscomplete, UE 115-b may perform uplink user/control plane duplicationwith first base station 105-c. For example, control plane data may beduplicated and shared between the first base station 105-c and thesecond base station 105-d. Accordingly, UE 115-b may achieve improvedreliability when receiving the data on the downlink. In some cases, theoperations at 350 through 360 may be referred to as a phase-II handoverexecution portion of a handover.

At 365, the second base station 105-d may transmit a role switchcomplete indication to the first base station 105-c. In some cases, therole switch complete indication may provide an indication to the firstbase station 105-c (e.g., via an Xn backhaul link) that the secondconnection has been established with the UE 115-b, and that signalingradio bearers (SRBs) and data radio bearers (DRBs) have been establishedin the second connection.

At 370, the first base station 105-c may transmit a SN status transferto the second base station 105-d. The SN status transfer may provide themost up to date PDCP status and downlink sequence number to use, forexample. The SN status transfer may also indicate to the second basestation 105-d that the first connection with the first base station105-c may be released in accordance with the DC handover procedure.

At 375, the second base station 105-d may make a handover completedecision and determine that the source first connection with the firstbase station 105-c may be released. At 380, the second base station105-d, responsive to determining that the handover is complete, maytransmit a RRC reconfiguration to the UE 115-b to release the firstconnection with the first base station 105-c. In some cases, the secondbase station 105-d may provide a handover completion indication to a UPFin the core network. Additionally or alternatively, the second basestation 105-d may transmit a PDCP status transfer to the UE 115-b.

At 385, the UE 115-b may release the first connection with the firstbase station 105-c. The UE 115-b may release the first connection, forexample, by removing DRBs and SRBs associated with the first basestation 105-c, and PDCP status and sequence numbers associated with thefirst connection. At 390, the UE 115-b may transmit a RRCreconfiguration complete indication to the second base station 105-d toindicate that the first connection has been released.

At 395, responsive to the indication that the first connection isreleased handover completion may be performed. In some cases, as part ofthe handover completion, the second base station 105-d may transmit apath switch request to an access and mobility function (AMF) of the corenetwork, and the AMF and UPF may perform a path switch related to corenetwork internal signaling to provide the actual downlink path used tothe UPF for subsequent downlink data transmissions. Further, in somecases, in the event that the first base station 105-c received freshdownlink data for the UE 115-b prior to the path switch at the UPF, thefirst base station 105-c may transmit the fresh downlink data to thesecond base station 105-d for downlink transmission to the UE 115-b. Insome cases, the AMF may provide an end marker to the first base station105-c to indicate that the downlink path has been changed to the secondbase station 105-d, and the first base station 105-c may provide anindication to the second base station 105-d that the end marker has beenreceived. In some cases, the AMF may provide a path switch requestacknowledgment to the second base station 105-d. Following the pathswitch acknowledgment, the second base station 105-d may transmit a UEcontext release command to the first base station 105-c, and the firstbase station 105-c may release UE context for the first connection andtransmit a UE context release complete back to the second base station105-d. In some cases, the operations at 365 through 395 may be referredto as a phase-III handover completion portion of a handover.

In some aspects, UE 115-b may use a dual protocol stack, which includesa source protocol stack for communicating with first base station 105-cand a target protocol stack for communicating with second base station105-d. Each of these protocol stacks may include a packet dataconvergence protocol (PDCP) layer, a radio link control (RLC) layer, amedium access control (MAC) layer, and/or a physical (PHY) layer. Insome aspects, the source protocol stack and the target protocol stackmay share one or more layers, such as a PDCP layer. In some aspects, thetarget protocol stack may be used for uplink data transmissions. Anexample of protocol stacks used at UEs 115 and base stations 105 invarious aspects of the disclosure is discussed with respect to FIG. 4.

FIG. 4 illustrates an example of dual connectivity handover protocolstacks 400 that supports handover techniques in wireless communicationsin accordance with aspects of the present disclosure. In some examples,dual connectivity handover 400 may implement aspects of wirelesscommunications system 100 or 200. The example DC handover protocolstacks 400 show network protocol stack and data flow for a DC handoverprocedure (e.g., similar to the handover procedure described withrespect to process flow 300 of FIG. 3).

In this example, a UE 415 may perform a DC handover between a source DU445 and a target DU 475. The UE 415 may be an example of UEs 115described herein, DUs 445 and 475 may be examples of DUs or basestations 105 described herein. In an initial setup phase 405-a in thisexample, a target gNB may include a target CU 490 and a target DU 475,and a source gNB may include a source CU 460 and source DU 445. Theinitial setup phase 405-a, which may correspond to the phase-I handoverpreparation phase, may provide setup at the target DU 475 and the targetCU 490.

In this example, an inter-gNB-CU handover is illustrated, and during theinitial setup phase 405-a the UE 415-a may use a source protocol stackfor a first connection, which may include a PDCP layer 420-a, a radiolink control (RLC) layer 425-a, and a medium access control layer 430-a,which the UE 415-a may use to communicate with a core network, such as a5GC 470-a via a source CU 460-a having PDCP layer 465-a and source DU445-a having MAC layer 450-a and RLC layer 455-a. The target gNB mayconfigure the target DU 475-a with MAC layer 480-a and RLC layer 485-a,and may configure the target CU 490-a with PDCH layer 490-a, inpreparation for the DC handover. The source DU 445-a may transmit a RRCreconfiguration to the UE 415-a (e.g., via a SRB), and the UE 415-a mayconfigure a DC RLC layer 435-a and MAC layer 440-a in preparation forrandom access channel (RACH) transmissions as part of a random accessprocedure to establish a second connection with the target DU 475-a. Insome cases, and the source CU 460-a may instruct the target CU 490-a(e.g., via an Xn backhaul interface 497-a) to configure the targetprotocol stack at the target CU 490-a and the target DU 475-a.

During a second SN status transfer phase 405-b, which may be an exampleof a phase-II handover execution phase, the target protocol stacks maybe used for communications with UE 415-b. As illustrated in FIG. 4, thetarget DU 475-b may communicate uplink data with the UE 415-b via PDCPlayer 420-b, RLC layer 435-b, and MAC layer 440-b. The target DU 475-bmay have an active and established MAC layer 480-b and RLC layer 485-b,which may be used for communication with the UE 415-b and target CU490-b that may have PDCP layer 495-b and may communicate with 5GC 470-b.In this example, the UE 415-b may maintain the protocol stack associatedwith the source DU 445-b, which may include PDCP layer 420-b, RLC layer425-b, and MAC layer 430-b. Likewise, the source gNB may maintain MAClayer 450-b and RLC layer 455-b at source DU 445-b, as well as PDCPlayer 465-b at source CU 460-b. In this example, uplink data may betransmitted using the target protocol stack, and downlink data may beforwarded via Xn backhaul interface 497-b from the source CU 460-b tothe target CU 490-b for transmission to the UE 415-b. In some aspects,the UE 415-b, source CU 460-b, target CU 490-b, and/or 5GC 470-b may usedifferent security keys corresponding to a same PDCP entity and/or mayhave different PDCP entities with a common reordering entity. In suchcases, the UE 415-b may decide on a security key (e.g., one or moresecurity keys from source CU 460-b or one or more security keys from atarget CU 490-b) to use based on the RLC/MAC/PHY stack from which datais received on the downlink or based on an explicit indication in thePDCP PDU header that indicates which security key to use.

During a third source release and path switch phase 405-c, which may bean example of a phase-III handover completion phase, the target protocolstacks may be used for communications with UE 415-c. As illustrated inFIG. 4, the target DU 475-c may communicate uplink/downlink data withthe UE 415-c via PDCP layer 420-c, RLC layer 435-c, and MAC layer 440-c.The target DU 475-c may have an active and established MAC layer 480-cand RLC layer 485-c, which may be used for communication with the UE415-c and target CU 490-c that may have PDCP layer 495-c and maycommunicate with 5GC 470-c. In this example, the UE 415-c, the source DU445-b and the source CU 460-b may release the corresponding sourceprotocol stacks. The remaining protocol stacks would then become sourceprotocol stacks in a subsequent handover procedure. It is noted thatFIG. 4 is provided as an example, and that other example protocol stacksare possible and may differ from what was described with respect to FIG.4.

FIG. 5 illustrates an example of a process flow 500 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. In some examples, process flow 500may implement aspects of wireless communications system 100 or 200. Asdiscussed herein, in some cases a UE or base station may experienceradio link issues during a handover procedure. For example, a UE mayexperience fast fading of a signal from a source base station or targetbase station that may result in the associated signal being unusable bythe UE. In such cases, if the UE can establish or maintain a connectionwith the other base station, an indication of the link failure may beprovided as part of the handover procedure.

The process flow 500 may include a source gNB DU 105-e, a target gNB DU105-f, a CU 505, and a UE 115-c. Source gNB DU 105-e and target gNB DU105-f may be examples of DU, a TRP, or a generally a base station 105 asdescribed herein. UE 115-c may be an example of corresponding UE devicesdescribed herein, and CU 505 may be an example of a CU, an ANC, orgenerally a base station 105 as described herein. In some examples, theprocess flow 500 may implement aspects of the wireless communicationssystem 100 and 200. For example, the source gNB DU 105-e, the target gNBDU 105-f, CU 505, and the UE 115-c, may support 0 ms or almost 0 msinterruption handovers.

In the following description of the process flow 500, the operationsbetween the source gNB DU 105-e, target gNB DU 105-f, CU 505, and the UE115-c may be transmitted in a different order than the exemplary ordershown, or the operations performed by the DUs 105 and the UE 115-c maybe performed in different orders or at different times. Certainoperations may also be left out of the process flow 500, or otheroperations may be added to the process flow 500.

In this example a handover between the source gNB DU 105-e and thetarget gNB DU 105-f is an enhanced make-before-break intra-CU handoverin which CU 505 is associated with both the source gNB DU 105-e and thetarget gNB DU 105-f. In some examples, the process flow 500 may commenceat 510 with the source gNB DU 105-e establishing a first connection withthe UE 115-c (e.g., performing a cell acquisition procedure, a randomaccess procedure, an RRC connection procedure, an RRC configurationprocedure, etc.).

At 515, an event trigger may occur that may cause the UE 115-c toperform a measurement procedure. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-c to identify one or more neighboring base stations thatmay be candidates for the handover procedure. In some cases, the eventtrigger may be a signal measurement of the source gNB DU 105-e droppingbelow a threshold value, may be a time-based periodic event, orcombinations thereof. At 520, the UE 115-c may transmit measurementreport to the source gNB DU 105-e, and the measurement report mayindicate that the target gNB DU 105-f is a candidate for a handover.

At 525, the source gNB DU 150-e may transmit a uplink RRC transfermessage to the CU 505. In some cases, the uplink RRC transfer messagemay include the measurement report from the UE 115-c, and may indicatethat the target gNB DU 105-f is a candidate for a handover.

At 530, the CU 515 may make a handover decision to handover the UE 115-cfrom the source gNB DU 105-e to the target gNB DU 105-f In some cases,the CU 505 may make the handover decision based on the measurementreport provided by the UE 115-c, one or more threshold values forinitiating a handover, a differential in one or more signal measurementsfrom one or more prior measurement reports, one or more measurementreports from other UEs, or any combinations thereof. In the example ofFIG. 5, the CU 505 may determine that the UE 115-c is to be handed overto the target gNB DU 105-f. Further, the CU 505 may determine that theUE 115-c has a capability of simultaneous transmissions and receptionsand may make a decision to perform an enhanced make-before-break basedhandover of the UE 115-c. In some cases, the CU 505 determines ahandover configuration that is to be used for the handover procedure forUE 115-c. For example, CU 505 may select from possible handoverprocedures that may be performed by UE 115-c based at least in part onthe indicated capability of UE 115-c. In the example of FIG. 5, CU 505may select an enhanced make-before-break handover procedure for UE 115-cbased at least in part on UE 115-c indicating a capability forsimultaneous transmissions and receptions that may be used in anenhanced make-before-break handover. In other examples, the CU 505 mayselect a DC handover procedure or a different handover procedure (e.g.,a legacy handover procedure), and different operations associated withthe selected handover procedure may be performed.

At 535, the CU and the target gNB DU 105-f may perform handoverpreparation. In some cases, handover preparation may include a UEcontext setup request that is transmitted from the CU 505 to the targetgNB DU 105-f, and an associated UE context setup response from thetarget gNB DU 105-f when the UE context has been configured.

At 540, the CU 505 may transmit a downlink RRC transfer to the sourcegNB DU 105-e, that may indicate an RRC reconfiguration. In some cases,the downlink RRC transfer may indicate that an enhancedmake-before-break handover is to be performed, and may provide a cell IDof the target gNB DU 105-f.

At 545, the source gNB DU 105-e may transmit a RRC reconfigurationmessage to the UE 115-c. The RRC reconfiguration message may indicate tothe UE 115-c that a connection with the target gNB DU 105-f is to beestablished according to an enhanced make-before-break handoverprocedure. For example, the RRC reconfiguration may indicate that UE115-c is to perform the make-before-break handover procedure with targetgNB DU 105-f using the simultaneous transmission/reception capability ofUE 115-c. In some aspects, the RRC reconfiguration can includeinformation identifying target gNB DU 105-f, information identifying ahandover configuration, an indication of the enhanced make-before-breakhandover procedure, or combinations thereof. In some cases, theoperations at 510 through 545 may be referred to as a phase-I handoverpreparation portion of a handover.

Following the transmission of the RRC reconfiguration at 545 and until asecond connection with the target gNB DU 105-f is established, downlinkdata that is to be transmitted to the UE 115-c may be provided to thesource gNB DU 105-e (e.g., from a user plane function (UPF) at the corenetwork), and the source gNB DU 105-e may transmit the downlink data tothe UE 115-c via the first connection. Similarly, uplink datatransmitted from the UE 115-c may be transmitted to the source gNB DU105-e via the first connection.

During the establishment of the second connection with the target gNB DU105-f, the UE 115-c may maintain the first connection with the sourcegNB DU 105-e, and thus have an active established connection during thehandover, as indicated at 575.

At 550, the UE 115-c and the target gNB DU 105-f may perform a randomaccess procedure to establish the second connection with the target gNBDU 105-f The random access procedure may be performed according toestablished random access procedures (e.g., random access channel (RACH)procedures that are established in LTE or NR).

At 555, it may be determined that the UE 115-c failed to connect to thetarget gNB DU 105-f, or that the second connection failed after beingestablished. In some cases, the failure may occur prior to completion ofthe random access procedure, or following completion of the randomaccess procedure. In cases where the failure is prior to completing therandom access procedure, for example, the UE 115-c may determine that atimer (e.g., a T304 timer in LTE or NR) associated with receipt of arandom access response message from the target gNB DU 105-f may expire,and thus the UE 115-c may determine that a handover failure hasoccurred. In cases where the failure is identified after completion ofthe random access procedure, the failure may be determined based on anumber of retransmissions of a message to the target gNB DU 105-f, orbased on a timer (e.g., a T310 timer in LTE or NR) associated with a RLClayer at the UE 115-c.

At 560, the UE 115-c may transmit a radio link failure (RLF) indicationto the source gNB DU 105-e, that may indicate a cause of the RLF asbeing a handover failure (e.g., a T304 expiry). In some cases, the RLFindication may preempt the UE 115-c from declaring RLF, and thus the UE115-c does not trigger an RRC reestablishment, due to the firstconnection remaining active. In cases where the UE 115-c detects RLF onthe target gNB DU 105-f after successfully connected and while stillconnected to source gNB DU 105-e as part of the handover procedure, UE115-c does not declare RLF and will not trigger RRC reestablishment asthe first connection is active. In such cases where the secondconnection fails after the establishment of the connection, the RLFindication at 560 may indicate a failure cause (e.g., T310 expiry,RandomAccessProblem, or RLCMaxReTx), the target cell group ID, or both.In some cases, the UE 115-c may also include one or more measurements(e.g., in a MeasResultHO-Failure indication) available for the targetgNB DU 105-e.

At 565, the source gNB DU 105-e may transmit a uplink RRC transfermessage to the CU 505. In some cases, the uplink RRC transfer messagemay include a RLF information or cause indication to the CU 505.Further, in cases where the UE 115-c provides one or more measurementsassociated with the target gNB DU 105-e, the source gNB DU 105-e mayinclude them with the uplink RRC transfer message.

At 570, the UE 115-c and the source gNB DU 105-e may continueuplink/downlink transmissions using the first connection. While theexample of FIG. 5 illustrates a make-before-break handover, similartechniques may be used in a DC handover to report a failure of aconnection at the source or target. Further, similar techniques ofreporting a connection failure may be used for both inter-gNB-CUhandovers and intra-gNB-CU handovers. Such techniques may allow the UE115-c to maintain the first connection even in the event that the secondconnection fails or is not able to be established. Thus, rather thanreporting a radio link failure and RRC reestablishment, the UE 115-c maysimply continue with the first connection, and thus a serviceinterruption is avoided. In some cases, the source gNB-DU 105-e mayrepeat operations starting at 525 based on the RLF indication.

In some cases, the CU 505 may repeat operations starting at 530 based onthe uplink RRC transfer message. For example, the CU 505 use the RLFinformation to identify the handover failure and may release the UEresources/context on the target gNB DU 105-f (e.g., a connection releaseat the target gNB in cases of intra-gNB-CU handover, or a handovercontext cancel with the target gNB-CU in cases of inter-gNB-CUhandover). In some cases, the CU 505 may use this information to triggeran immediate handover to another target cell by sending a new downlinkRRC transfer (e.g., a new RRC reconfiguration message) with the newtarget cell information.

In some cases, the UE 115-c may initiate the establishment of the secondconnection with the target gNB DU 105-f, and the first connection withthe source gNB DU 105-e may experience a RLF. In such cases, if the UE115-c detects RLF on source gNB DU 105-e while performing the randomaccess procedure at the target gNB DU 105-f, or after successfullyconnected to target gNB DU 105-f, the UE 115-c may continue the randomaccess procedure without triggering a RRC reestablishment. The UE 115-cmay then release the source stack and the source connection implicitly,and ignores any RRC reconfiguration to release the first connection.Additionally or alternatively, the UE 115-c in such cases, may provide aRLF indication (e.g., that indicates RLF on source gNB DU 105-e and thesource cell group ID) to the target gNB DU 105-f, which may be providedto the CU 505. The CU 505 may use this information to release the UE115-c resources/context on the source gNB DU 105-e (or source DU in caseof Intra-gNB-CU HO and with the source gNB-CU/source gNB-DU in case ofInter-gNB-CU HO). In cases where the second connection fails after thefirst connection is released, the UE 115-c may trigger RRCreestablishment.

FIG. 6 illustrates an example of a process flow 600 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. In some examples, process flow 600may implement aspects of wireless communications system 100 or 200.Similarly as discussed with respect to FIG. 5, in this example a UE orbase station may experience radio link issues during a handoverprocedure (e.g., a UE may experience fast fading of a signal from asource base station or target base station that may result in theassociated signal being unusable by the UE). In this example, an exampleof an inter-gNB-CU enhanced make-before-break handover is illustrated.

The process flow 600 may include a source base station 105-g, a targetbase station 105-h, and a UE 115-d. Source base station 105-g and targetbase station 105-h may be examples of a base station 105 as describedherein, and in some cases may include one or more associated DUs orTRPs, and a CU or ANC. UE 115-d may be an example of corresponding UEdevices described herein. In some examples, the process flow 600 mayimplement aspects of the wireless communications system 100 and 200. Forexample, the source base station 105-g, the target base station 105-h,and the UE 115-d, may support 0 ms or almost 0 ms interruptionhandovers.

In the following description of the process flow 600, the operationsbetween the source base station 105-g, target base station 105-h, andthe UE 115-d may be transmitted in a different order than the exemplaryorder shown, or the operations performed by the CUs 105 and the UE 115-dmay be performed in different orders or at different times. Certainoperations may also be left out of the process flow 600, or otheroperations may be added to the process flow 600.

In this example a handover between the source base station 105-g and thetarget base station 105-h, as indicated above, is an enhancedmake-before-break inter-CU handover in which both the source basestation 105-g and the target base station 105-h have differentassociated CUs. In some examples, the process flow 600 may commence at610 with the source base station 105-g establishing a first connectionwith the UE 115-d (e.g., performing a cell acquisition procedure, arandom access procedure, an RRC connection procedure, an RRCconfiguration procedure, etc.).

At 615, an event trigger may occur that may cause the UE 115-d toperform a measurement procedure. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-d to identify one or more neighboring base stations thatmay be candidates for the handover procedure. In some cases, the eventtrigger may be a signal measurement of the source base station 105-gdropping below a threshold value, may be a time-based periodic event, orcombinations thereof. At 620, the UE 115-d may transmit measurementreport to the source base station 105-g, and the measurement report mayindicate that the target base station 105-h is a candidate for ahandover.

At 625, the source base station 105-g (e.g., a CU associated with thesource base station 105-g) may make a handover decision to handover theUE 115-d from the source base station 105-g to the target base station105-h. In some cases, the source base station 105-g may make thehandover decision based on the measurement report provided by the UE115-d, one or more threshold values for initiating a handover, adifferential in one or more signal measurements from one or more priormeasurement reports, one or more measurement reports from other UEs, orany combinations thereof. In the example of FIG. 6, the source basestation 105-g may determine that the UE 115-d is to be handed over tothe target base station 105-h. Further, the source base station 105-gmay determine that the UE 115-d has a capability of simultaneoustransmissions and receptions and may make a decision to perform anenhanced make-before-break based handover of the UE 115-d. In somecases, the source base station 105-g determines a handover configurationthat is to be used for the handover procedure for UE 115-d. For example,source base station 105-g may select from possible handover proceduresthat may be performed by UE 115-d based at least in part on theindicated capability of UE 115-d. In the example of FIG. 6, source basestation 105-g may select an enhanced make-before-break handoverprocedure for UE 115-d based at least in part on UE 115-d indicating acapability for simultaneous transmissions and receptions that may beused in an enhanced make-before-break handover. In other examples, thesource base station 105-g may select a DC handover procedure or adifferent handover procedure (e.g., a legacy handover procedure), anddifferent operations associated with the selected handover procedure maybe performed.

At 630, the source base station 105-g may transmit a handover request tothe target base station 105-h. In some cases, the handover request mayindicate that the target base station 105-h is a selected for a handoverof UE 115-d. In some cases, the handover request may also include themeasurement report from the UE 115-d.

At 635, the target base station 105-h (e.g., a CU associated with thetarget base station 105-h) may perform admission control, and maydetermine that the second base station 105-h can accommodate thehandover. In some cases, admission control may include a UE contextsetup. At 640, the target base station 105-h may transmit a handoverrequest acknowledgment to the source base station 105-g.

At 645, the source base station 105-g may transmit a RRC reconfigurationmessage to the UE 115-d. The RRC reconfiguration message may indicate tothe UE 115-d that a connection with the target base station 105-h is tobe established according to an enhanced make-before-break handoverprocedure. For example, the RRC reconfiguration may indicate that UE115-d is to perform the make-before-break handover procedure with targetbase station 105-h using the simultaneous transmission/receptioncapability of UE 115-d. In some aspects, the RRC reconfiguration caninclude information identifying target base station 105-h, informationidentifying a handover configuration, an indication of the enhancedmake-before-break handover procedure, or combinations thereof. In somecases, the operations at 610 through 645 may be referred to as a phase-Ihandover preparation portion of a handover.

Following the transmission of the RRC reconfiguration at 645 and until asecond connection with the target base station 105-h is established,downlink data that is to be transmitted to the UE 115-d may be providedto the source base station 105-g (e.g., from a user plane function (UPF)at the core network), and the source base station 105-g may transmit thedownlink data to the UE 115-d via the first connection. Similarly,uplink data transmitted from the UE 115-d may be transmitted to thesource base station 105-g via the first connection.

During the establishment of the second connection with the target basestation 105-h, the UE 115-d may maintain the first connection with thesource base station 105-g, and thus have an active establishedconnection during the handover, as indicated at 675.

At 650, the UE 115-d and the target base station 105-h may perform arandom access procedure to establish the second connection with thetarget base station 105-h. The random access procedure may be performedaccording to established random access procedures (e.g., random accesschannel (RACH) procedures that are established in LTE or NR).

At 655, it may be determined that the UE 115-d failed to connect to thetarget base station 105-h, or that the second connection failed afterbeing established. In some cases, the failure may occur prior tocompletion of the random access procedure, or following completion ofthe random access procedure. In cases where the failure is prior tocompleting the random access procedure, for example, the UE 115-d maydetermine that a timer (e.g., a T304 timer in LTE or NR) associated withreceipt of a random access response message from the target base station105-h may expire, and thus the UE 115-d may determine that a handoverfailure has occurred. In cases where the failure is identified aftercompletion of the random access procedure, the failure may be determinedbased on a number of retransmissions of a message to the target basestation 105-h, or based on a timer (e.g., a T310 timer in LTE or NR)associated with a RLC layer at the UE 115-d.

At 660, the UE 115-d may transmit a radio link failure (RLF) indicationto the source base station 105-g, that may indicate a cause of the RLFas being a handover failure (e.g., a T304 expiry). In some cases, theRLF indication may preempt the UE 115-d from declaring RLF, and thus theUE 115-d does not trigger an RRC reestablishment, due to the firstconnection remaining active. In cases where the UE 115-d detects RLF onthe target base station 105-h after successfully connected and whilestill connected to source base station 105-g as part of the handoverprocedure, UE 115-d does not declare RLF and will not trigger RRCreestablishment as the first connection is active. In such cases wherethe second connection fails after the establishment of the connection,the RLF indication at 660 may indicate a failure cause (e.g., T310expiry, RandomAccessProblem, or RLCMaxReTx), the target cell group ID,or both. In some cases, the UE 115-d may also include one or moremeasurements (e.g., in a MeasResultHO-Failure indication) available forthe target base station 105-g. At 665, the source base station 105-g maytransmit a handover cancel message to the target base station 105-h. Insome cases, the handover cancel message may include RLF information orcause indication.

At 670, the UE 115-d and the source base station 105-g may continueuplink/downlink transmissions using the first connection. While theexample of FIG. 6 illustrates a make-before-break handover, similartechniques may be used in an inter-gNB-CU DC handover to report afailure of a connection at the source or target, and may perform furtheractions such as discussed above with respect to FIG. 5.

FIG. 7 illustrates an example of a process flow 700 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. In some examples, process flow 700may implement aspects of wireless communications system 100 or 200. Insome cases, during a handover of a UE, channel conditions of the sourceand the target may be relatively close to one another (e.g., due to theUE being approximately a same distance away from a cell edge of both thesource and the target). In some cases, a hysteresis may be applied tochannel measurements for initiating a handover to prevent excessiveping-pong handovers between source and target base stations. However,such a hysteresis may result in a UE that is not taking advantage of abase station that may have better channel conditions and thus betterefficiency and reliability. In the example of FIG. 7, an example ofping-pong handling is discussed in which a UE may maintain connectionswith both the source and target and utilize a connection with betterchannel conditions, and may thus provide enhanced reliability andthroughput.

The process flow 700 may include a source base station 105-i, a targetbase station 105-j, and a UE 115-e. Source base station 105-i and targetbase station 105-j may be examples of a base station 105 as describedherein, and in some cases may include one or more associated DUs orTRPs, and a CU or ANC. UE 115-e may be an example of corresponding UEdevices described herein. In some examples, the process flow 700 mayimplement aspects of the wireless communications system 100 and 200. Forexample, the source base station 105-i, the target base station 105-j,and the UE 115-e, may support 0 ms or almost 0 ms interruptionhandovers.

In the following description of the process flow 700, the operationsbetween the source base station 105-i, target base station 105-j, andthe UE 115-e may be transmitted in a different order than the exemplaryorder shown, or the operations performed by the CUs 105 and the UE 115-emay be performed in different orders or at different times. Certainoperations may also be left out of the process flow 700, or otheroperations may be added to the process flow 700.

In this example a handover between the source base station 105-i and thetarget base station 105-j, as indicated above, is an enhancedmake-before-break inter-CU handover in which both the source basestation 105-i and the target base station 105-j have differentassociated CUs. In some examples, the process flow 700 may commence at710 with the source base station 105-i establishing a first connectionwith the UE 115-e (e.g., performing a cell acquisition procedure, arandom access procedure, an RRC connection procedure, an RRCconfiguration procedure, etc.).

At 715, an event trigger may occur that may cause the UE 115-e toperform a measurement procedure. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-e to identify one or more neighboring base stations thatmay be candidates for the handover procedure. In some cases, the eventtrigger may be a signal measurement of the source base station 105-idropping below a threshold value, may be a time-based periodic event, orcombinations thereof. At 720, the UE 115-e may transmit measurementreport to the source base station 105-i, and the measurement report mayindicate that the target base station 105-j is a candidate for ahandover.

At 725, the source base station 105-i (e.g., a CU associated with thesource base station 105-i), the target base station 105-j (e.g., a CUassociated with the target base station 105-j), and the UE 115-e mayperform an enhanced make-before-break handover procedure, such asdiscussed above, and may establish a second connection between the UE115-e and the target base station 105-j. Once established, the secondconnection may be the primary connection for the UE 115-e, and the firstconnection may be the secondary connection of the UE 115-e. In somecases, the UE 115-e may maintain both the first connection and thesecond connection, as indicated at 770, until one of the connections isrelatively stable and provides a better channel for a certain period oftime.

In this example, the UE 115-e, source base station 105-i, and targetbase station 105-j, may perform ping-pong handling 730, in which aprimary connection of the UE 115-e may be switched based on which of thefirst connection or second connection has more favorable channelconditions.

At 735, while the UE 115-e is connected to both the source base station105-i and the target base station 105-j, an event trigger may occur thatmay cause the UE 115-e to perform a measurement procedure. Such ameasurement procedure may include, for example, signal measurements ofthe source base station 105-i, the target base station 105-j, and/or oneor more other neighboring base stations. In some cases, the eventtrigger may be a signal measurement of the source base station 105-i. Insome cases, the event trigger may be when the secondary connection has asignal strength that exceeds the primary connection signal strength by athreshold value.

At 740, the UE 115-e may transmit measurement report to the target basestation 105-j (i.e., to the base station 105 that is currently providingthe primary connection), and the measurement report may indicate thatthe source base station 105-i has better channel conditions than thetarget base station 105-j.

At 745, based on the measurement report, the target base station 105-jmay transmit a handover switch request to the source base station 105-i,which requests to switch the primary connection to the source basestation 105-i, and the secondary connection to be the connection withthe target base station 105-j. At 750, the source base station 105-i maytransmit a handover switch acknowledgment that indicates that the sourcebase station 105-i is prepared to provide the primary connection withthe UE 115-e.

At 755, the target base station 105-j may transmit a RRC reconfigurationto the UE 115-e to switch the primary and secondary connections. The UE115-e, based on the RRC reconfiguration, may reconfigure the primary andsecondary connections to swap the connections, and may, at 760, transmita RRC reconfiguration complete indication to the source base station105-i. In some cases, the indication to switch the primary and secondaryconnections may be provided to the UE 115-e via a MAC CE.

The operations described at 735 through 760 may be repeated with theswapped primary and secondary connections until, at 765, it isdetermined that one of the first connection or second connection hasserved as the primary connection for a duration of a ping-pong timer. Insome cases, a duration of the ping-pong timer may be determined based onchannel conditions at the UE 115-e, a predetermined duration, a historyof ping-pong procedure durations, or any combinations thereof. At 775,the secondary connection may be released, and communication may continueusing the primary connection until a subsequent handover procedure isinitiated.

Such ping-pong handling 730 may be useful in certain deployments, suchas deployments that use mmW or in inter-frequency handover scenarios,where the source base station 105-i may decide to initiate the handoverto the target base station 105-j much earlier than the actual need forhandover to reduce the possibility for handover command failure. In suchcases, once the target base station 105-j link is established, it ispossible that the target link becomes weaker than source and ping-ponghappens. Thus, such techniques may be beneficial to maintain the sourcebase station 105-i connection for some time until the target basestation 105-j link is good enough to maintain the primary connection. Bymaintaining the source base station 105-i connection, the target basestation 105-j may handle ping-pong efficiently through a RRCReconfiguration without the need to perform a random access procedure orother backend procedures on the source base station 105-j again. Whilethe techniques of FIG. 7 show enhanced make-or-break handover, suchtechniques may also be used for DC based handover.

In some cases, SRB/DRB data can be transmitted/received by the UE 115-eonly on the primary link, or can be duplicated over both the linksduring the ping-pong handling procedure at 730. In cases whereduplication of data is provided, which may enhance reliability ofcommunications, additional configuration information may be exchangedover a backhaul link (e.g., an Xn link) between the source base station105-i and the target base station 105-jm to establish backhaulduplication and the tunnel information. Further, uplink duplicated datasent by the UE 115-e may be forwarded over the backhaul link to theprimary connection for forwarding to the UPF. In such cases, downlinkdata is still received by the original source base station 105-i (due tothe core network path switch not happening until a stable link isdecided), and forwarded to the other base station via backhaul link forduplication of downlink data on the target base station 105-j link. Insome cases, the UE 115-e may store both base station keys and use thekey based on the transmission/reception connection. Such techniques maybe used for both intra-gNB-CU and inter-gNB-CU handover. In the case ofintra-gNB-CU handover, the switch may be controlled by the gNB-CU tomove between the cells under different DUs.

FIG. 8 illustrates an example of a process flow 800 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. In some examples, process flow 800may implement aspects of wireless communications system 100 or 200. Insome cases, a UE may perform a measurement that may indicate that aneighboring base station has a better channel condition than a source ortarget base station. In some cases, the UE may provide a measurementreport that indicates the neighboring base station, and may trigger ahandover abort of a handover to the target base station in favor of theneighboring base station with better channel conditions. Such handoverabort may allow the UE to establish a connection with a more favorablebase station and thereby enhance reliability and communications in awireless communications system. In this example, an example of aninter-gNB-CU enhanced make-before-break handover is illustrated,although such techniques may be used for DC handover, intra-gNB-CUhandovers, or any combinations thereof.

The process flow 800 may include a source base station 105-k, a firsttarget base station 105-l, a second target base station 105-m, and a UE115-f. Source base station 105-k, first target base station 105-l, andsecond target base station 105-m may be examples of a base station 105as described herein, and in some cases may include one or moreassociated DUs or TRPs, and a CU or ANC. UE 115-f may be an example ofcorresponding UE devices described herein. In some examples, the processflow 800 may implement aspects of the wireless communications system 100and 200. For example, the source base station 105-k, the target basestations 105, and the UE 115-f, may support 0 ms or almost 0 msinterruption handovers.

In the following description of the process flow 800, the operationsbetween the source base station 105-k, first target base station 105-l,second target base station 105-m and the UE 115-f may be transmitted ina different order than the exemplary order shown, or the operationsperformed by the base stations 105 and the UE 115-f may be performed indifferent orders or at different times. Certain operations may also beleft out of the process flow 800, or other operations may be added tothe process flow 800.

In this example a handover between the source base station 105-k and thetarget base station 105-l, as indicated above, is an enhancedmake-before-break inter-CU handover in which both the source basestation 105-k and the target base station 105-l have differentassociated CUs. In some examples, the process flow 800 may commence at810 with the source base station 105-k establishing a first connectionwith the UE 115-f (e.g., performing a cell acquisition procedure, arandom access procedure, an RRC connection procedure, an RRCconfiguration procedure, etc.).

At 815, an event trigger may occur that may cause the UE 115-f toperform a measurement procedure. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-f to identify one or more neighboring base stations thatmay be candidates for the handover procedure. In some cases, the eventtrigger may be a signal measurement of the source base station 105-kdropping below a threshold value, may be a time-based periodic event, orcombinations thereof. At 820, the UE 115-f may transmit measurementreport to the source base station 105-k, and the measurement report mayindicate that the first target base station 105-l is a candidate for ahandover.

At 825, the source base station 105-k (e.g., a CU associated with thesource base station 105-k) may make a handover decision to handover theUE 115-f from the source base station 105-k to the first target basestation 105-l. In some cases, the source base station 105-k may make thehandover decision based on the measurement report provided by the UE115-f, one or more threshold values for initiating a handover, adifferential in one or more signal measurements from one or more priormeasurement reports, one or more measurement reports from other UEs, orany combinations thereof. In the example of FIG. 8, the source basestation 105-k may determine that the UE 115-f is to be handed over tothe first target base station 105-l. Further, the source base station105-k may determine that the UE 115-f has a capability of simultaneoustransmissions and receptions and may make a decision to perform anenhanced make-before-break based handover of the UE 115-f. In somecases, the source base station 105-k determines a handover configurationthat is to be used for the handover procedure for UE 115-f. For example,source base station 105-k may select from possible handover proceduresthat may be performed by UE 115-f based at least in part on theindicated capability of UE 115-f In the example of FIG. 8, source basestation 105-k may select an enhanced make-before-break handoverprocedure for UE 115-f based at least in part on UE 115-f indicating acapability for simultaneous transmissions and receptions that may beused in an enhanced make-before-break handover. In other examples, thesource base station 105-k may select a DC handover procedure or adifferent handover procedure (e.g., a legacy handover procedure), anddifferent operations associated with the selected handover procedure maybe performed.

At 830, the source base station 105-k may transmit a handover request tothe first target base station 105-l. In some cases, the handover requestmay indicate that the first target base station 105-l is a selected fora handover of UE 115-f. In some cases, the handover request may alsoinclude the measurement report from the UE 115-f.

At 835, the first target base station 105-l (e.g., a CU associated withthe target base station 105-l) may perform admission control, and maydetermine that the first target base station 105-l can accommodate thehandover. In some cases, admission control may include a UE contextsetup. At 840, the first target base station 105-l may transmit ahandover request acknowledgment to the source base station 105-k.

At 845, the source base station 105-k may transmit a RRC reconfigurationmessage to the UE 115-f The RRC reconfiguration message may indicate tothe UE 115-f that a connection with the first target base station 105-lis to be established according to an enhanced make-before-break handoverprocedure. For example, the RRC reconfiguration may indicate that UE115-f is to perform the make-before-break handover procedure with firsttarget base station 105-l using the simultaneous transmission/receptioncapability of UE 115-f. In some aspects, the RRC reconfiguration caninclude information identifying first target base station 105-l,information identifying a handover configuration, an indication of theenhanced make-before-break handover procedure, or combinations thereof.

Following the transmission of the RRC reconfiguration at 845 and until asecond connection with the first target base station 105-l isestablished, downlink data that is to be transmitted to the UE 115-f maybe provided to the source base station 105-k (e.g., from a user planefunction (UPF) at the core network), and the source base station 105-kmay transmit the downlink data to the UE 115-f via the first connection.Similarly, uplink data transmitted from the UE 115-f may be transmittedto the source base station 105-k via the first connection.

During the establishment of the second connection with the first targetbase station 105-l, the UE 115-f may maintain the first connection withthe source base station 105-k, and thus have an active establishedconnection during the handover, as indicated at 875.

At 850, the UE 115-f and the first target base station 105-l may performa random access procedure to establish the second connection with thefirst target base station 105-l. The random access procedure may beperformed according to established random access procedures (e.g.,random access channel (RACH) procedures that are established in LTE orNR). In the example of FIG. 8, the UE 115-f may continue to performneighbor cell measurements while acquiring the first target base station105-k.

Such neighbor cell measurements may be initiated at 860, where an eventtrigger may occur that may cause the UE 115-f to perform a measurementprocedure for neighbor cells, which may include a measurement of thesecond target base station 105-m. Such a measurement procedure mayinclude, for example, signal measurements of neighboring base stationsby the UE 115-f to identify one or more neighboring base stations thatmay be better candidates for handover than the first target base station105-l. In some cases, the event trigger may be a signal measurement ofthe first target base station 105-l dropping below a threshold value,may be a time-based periodic event, or combinations thereof. At 865, theUE 115-f may transmit measurement report to the source base station105-k, and the measurement report may indicate that the second targetbase station 105-m has better channel conditions than the first targetbase station 105-l.

At 870, the source base station 105-k (e.g., a CU associated with thesource base station 105-k) may make a handover decision to handover theUE 115-f from the source base station 105-k to the second target basestation 105-m. In some cases, the source base station 105-k may make thehandover decision based on the measurement report provided by the UE115-f, one or more threshold values for aborting the handover to thefirst target base station 105-l, a differential in one or more signalmeasurements from one or more prior measurement reports, one or moremeasurement reports from other UEs, or any combinations thereof. In theexample of FIG. 8, the source base station 105-k may determine that thehandover to the first target base station 105-l is to be aborted infavor of a handover to the second target base station 105-m.

At 870, the source base station 105-k may transmit a handover request tothe second target base station 105-m. In some cases, the handoverrequest may indicate that the second target base station 105-m is aselected for a handover of UE 115-f. In some cases, the handover requestmay also include the measurement report from the UE 115-f.

At 875, the second target base station 105-m (e.g., a CU associated withthe target base station 105-l) may perform admission control, and maydetermine that the second target base station 105-m can accommodate thehandover. In some cases, admission control may include a UE contextsetup. At 880, the second target base station 105-m may transmit ahandover request acknowledgment to the source base station 105-k.

At 885, the source base station 105-k may transmit a RRC reconfigurationmessage to the UE 115-f The RRC reconfiguration message may indicate tothe UE 115-f that the handover with the first target base station 105-lis to be aborted in favor of a handover to the second target basestation 105-m. Following the transmission of the RRC reconfiguration at885, the source base station 105-k may transmit a handover cancelindication to the first target base station 105-l, at indicated at 890.

At 895, the UE 115-f and the second target base station 105-m mayperform a random access procedure to establish a second connection withthe second target base station 105-m. The UE 115-m, the first targetbase station 105-k, and the second target base station 105-m may followenhanced make-or-break handover procedures such as discussed herein(e.g., connection establishment, radio link failures associated with aconnection, handover abort, etc.).

In some cases, the UE 115-f may treat the source base station 105-k as aPcell and the first target base station 105-l as the Scell, and may sendmeasurement reports (e.g., an A3 and A6 measurement report in an LTE orNR system). Based on the measurement reports, the source base station105-k or associated CU may consider that another better neighbor cell isdetected if the A3 and A6 measurement reports are received withincertain time from the UE. In other cases, a new measurement event (e.g.,an A36 measurement event) may be defined, which is to report when aneighbor cell is detected to be stronger than the source base station105-k and the first target base station 105-l.

FIG. 9 shows a block diagram 900 of a device 905 that supports handovertechniques in wireless communications in accordance with aspects of thepresent disclosure. The device 905 may be an example of aspects of a UE115 as described herein. The device 905 may include a receiver 910, acommunications manager 915, and a transmitter 920. The device 905 mayalso include a processor. Each of these components may be incommunication with one another (e.g., via one or more buses).

The receiver 910 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to handovertechniques in wireless communications, etc.). Information may be passedon to other components of the device 905. The receiver 910 may be anexample of aspects of the transceiver 1220 described with reference toFIG. 12. The receiver 910 may utilize a single antenna or a set ofantennas.

In some cases, the communications manager 915 may establish a firstconnection with a first base station, receive a handover message fromthe first base station to perform a handover procedure with a secondbase station, transmit, responsive to the handover message, a request tothe second base station to establish a second connection with the secondbase station, where the first connection with the first base station ismaintained during the handover procedure, determine that a radio linkfailure of the first connection or the second connection has occurred,and transmit an indication of the radio link failure to the first basestation or the second base station responsive to the determining.

In some cases, the communications manager 915 may also establish a firstconnection with a first base station, receive a handover message fromthe first base station to perform a handover procedure with a secondbase station, initiate a connection establishment with the second basestation to establish a second connection responsive to the handovermessage, where the first connection is maintained during the connectionestablishment with the second base station, measure one or more channelconditions associated with one or more of the first base station, thesecond base station or a third base station, and transmit, responsive tothe handover message, a measurement report to at least one of the firstbase station or the second base station responsive to measuring the oneor more channel conditions. The communications manager 915 may be anexample of aspects of the communications manager 1210 described herein.

The communications manager 915, or its sub-components, may beimplemented in hardware, code (e.g., software or firmware) executed by aprocessor, or any combination thereof. If implemented in code executedby a processor, the functions of the communications manager 915, or itssub-components may be executed by a general-purpose processor, a DSP, anapplication-specific integrated circuit (ASIC), a FPGA or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described in the present disclosure.

The communications manager 915, or its sub-components, may be physicallylocated at various positions, including being distributed such thatportions of functions are implemented at different physical locations byone or more physical components. In some examples, the communicationsmanager 915, or its sub-components, may be a separate and distinctcomponent in accordance with various aspects of the present disclosure.In some examples, the communications manager 915, or its sub-components,may be combined with one or more other hardware components, includingbut not limited to an input/output (I/O) component, a transceiver, anetwork server, another computing device, one or more other componentsdescribed in the present disclosure, or a combination thereof inaccordance with various aspects of the present disclosure.

The transmitter 920 may transmit signals generated by other componentsof the device 905. In some examples, the transmitter 920 may becollocated with a receiver 910 in a transceiver module. For example, thetransmitter 920 may be an example of aspects of the transceiver 1220described with reference to FIG. 12. The transmitter 920 may utilize asingle antenna or a set of antennas.

FIG. 10 shows a block diagram 1000 of a device 1005 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The device 1005 may be an example ofaspects of a device 905, or a UE 115 as described herein. The device1005 may include a receiver 1010, a communications manager 1015, and atransmitter 1045. The device 1005 may also include a processor. Each ofthese components may be in communication with one another (e.g., via oneor more buses).

The receiver 1010 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to handovertechniques in wireless communications, etc.). Information may be passedon to other components of the device 1005. The receiver 1010 may be anexample of aspects of the transceiver 1220 described with reference toFIG. 12. The receiver 1010 may utilize a single antenna or a set ofantennas.

The communications manager 1015 may be an example of aspects of thecommunications manager 915 as described herein. The communicationsmanager 1015 may include a connection establishment manager 1020, ahandover manager 1025, a random access manager 1030, a RLF manager 1035,and a measurement component 1040. The communications manager 1015 may bean example of aspects of the communications manager 1210 describedherein.

The connection establishment manager 1020 may establish a firstconnection with a first base station.

The handover manager 1025 may receive a handover message from the firstbase station to perform a handover procedure with a second base station.

The random access manager 1030 may transmit, responsive to the handovermessage, a request to the second base station to establish a secondconnection with the second base station, where the first connection withthe first base station is maintained during the handover procedure.

The RLF manager 1035 may determine that a radio link failure of thefirst connection or the second connection has occurred and transmit anindication of the radio link failure to the first base station or thesecond base station responsive to the determining.

The measurement component 1040 may measure one or more channelconditions associated with one or more of the first base station, thesecond base station or a third base station and transmit, responsive tothe handover message, a measurement report to at least one of the firstbase station or the second base station responsive to measuring the oneor more channel conditions.

The transmitter 1045 may transmit signals generated by other componentsof the device 1005. In some examples, the transmitter 1045 may becollocated with a receiver 1010 in a transceiver module. For example,the transmitter 1045 may be an example of aspects of the transceiver1220 described with reference to FIG. 12. The transmitter 1045 mayutilize a single antenna or a set of antennas.

FIG. 11 shows a block diagram 1100 of a communications manager 1105 thatsupports handover techniques in wireless communications in accordancewith aspects of the present disclosure. The communications manager 1105may be an example of aspects of a communications manager 915, acommunications manager 1015, or a communications manager 1210 describedherein. The communications manager 1105 may include a connectionestablishment manager 1110, a handover manager 1115, a random accessmanager 1120, a RLF manager 1125, a measurement component 1130, and a DCmanager 1135. Each of these modules may communicate, directly orindirectly, with one another (e.g., via one or more buses).

The connection establishment manager 1110 may establish a firstconnection with a first base station. In some examples, the connectionestablishment manager 1110 may determine that the first connection withthe first base station has failed prior to completion of anestablishment of the second connection.

In some examples, the connection establishment manager 1110 may completethe establishment of the second connection with the second base station.In some examples, the connection establishment manager 1110 may releasethe first connection and a source stack associated with the firstconnection responsive to completing the establishment of the secondconnection with the second base station. In some cases, the completingthe establishment of the second connection preempts a triggering of areestablishment of the first connection with the first base station inthe event of a RLF at the second connection.

The handover manager 1115 may receive a handover message from the firstbase station to perform a handover procedure with a second base station.In some examples, the handover manager 1115 may release the firstconnection after the second connection has been a primary connection fora predetermined time period. In some cases, a reconfiguration messageincludes a second handover message from the first base station toperform a second handover procedure with the third base station.

The random access manager 1120 may transmit, responsive to the handovermessage, a request to the second base station to establish a secondconnection with the second base station, where the first connection withthe first base station is maintained during the handover procedure.

The RLF manager 1125 may determine that a radio link failure of thefirst connection or the second connection has occurred. In someexamples, the RLF manager 1125 may transmit an indication of the radiolink failure to the first base station or the second base stationresponsive to the determining. In some examples, the RLF manager 1125may determine that establishment of the connection with the second basestation has failed or the second connection failed after establishmentof the second connection.

In some cases, the indication of the radio link failure includes afailure cause associated with the second connection and a target cellgroup identification associated with the second base station. In somecases, the indication of the radio link failure further indicates one ormore of a timer expiration associated with the second connection, arandom access procedure failure, or a maximum number of retransmissionsis reached for a communication using the second connection. In somecases, the indication of the radio link failure indicates a failurecause associated with the first connection and a source cell groupidentification associated with the first base station.

The measurement component 1130 may measure one or more channelconditions associated with one or more of the first base station, thesecond base station or a third base station. In some examples, themeasurement component 1130 may transmit, responsive to the handovermessage, a measurement report to at least one of the first base stationor the second base station responsive to measuring the one or morechannel conditions. In some examples, the measurement component 1130 maytransmit one or more additional measurement reports via the primaryconnection. In some cases, the indication of the radio link failurefurther indicates a measurement of one or more signals received at theUE from one or more neighboring base stations.

In some cases, the measurement report is transmitted responsive to themeasured one or more channel conditions of the secondary connectionbeing better than corresponding channel conditions of the primaryconnection. In some cases, the measurement report is transmittedresponsive to measured channel conditions of the third base stationbeing better than measured channel conditions of the second basestation.

The DC manager 1135 may establish the second connection with the secondbase station, where the second connection is a primary connection andthe first connection is maintained as a secondary connection.

In some examples, the DC manager 1135 may receive, responsive to themeasurement report, a reconfiguration message from the second basestation to reconfigure the first connection as the primary connectionand the second connection as the secondary connection. In some examples,the DC manager 1135 may reconfigure the first connection and the secondconnection responsive to the reconfiguration message. In some examples,the DC manager 1135 may transmit and receiving duplicated data via eachof the primary connection and the secondary connection. In some cases,the reconfiguring the first connection and the second connectionresponsive to the reconfiguration message is performed withoutperforming a random access procedure.

In some examples, the DC manager 1135 may receive, responsive to themeasurement report, a reconfiguration message from the first basestation to abort the handover procedure with the second base station.

FIG. 12 shows a diagram of a system 1200 including a device 1205 thatsupports handover techniques in wireless communications in accordancewith aspects of the present disclosure. The device 1205 may be anexample of or include the components of device 905, device 1005, or a UE115 as described herein. The device 1205 may include components forbi-directional voice and data communications including components fortransmitting and receiving communications, including a communicationsmanager 1210, an I/O controller 1215, a transceiver 1220, an antenna1225, memory 1230, and a processor 1240. These components may be inelectronic communication via one or more buses (e.g., bus 1245).

The communications manager 1210 may establish, at a UE, a firstconnection with a first base station, receive a handover message fromthe first base station to perform a handover procedure with a secondbase station, transmit, responsive to the handover message, a request tothe second base station to establish a second connection with the secondbase station, where the first connection with the first base station ismaintained during the handover procedure, determine that a radio linkfailure of the first connection or the second connection has occurred,and transmit an indication of the radio link failure to the first basestation or the second base station responsive to the determining. Thecommunications manager 1210 may also establish, at a UE, a firstconnection with a first base station, receive a handover message fromthe first base station to perform a handover procedure with a secondbase station, initiate, at the UE, a connection establishment with thesecond base station to establish a second connection responsive to thehandover message, where the first connection is maintained during theconnection establishment with the second base station, measure one ormore channel conditions associated with one or more of the first basestation, the second base station or a third base station, and transmit,responsive to the handover message, a measurement report to at least oneof the first base station or the second base station responsive tomeasuring the one or more channel conditions.

The I/O controller 1215 may manage input and output signals for thedevice 1205. The I/O controller 1215 may also manage peripherals notintegrated into the device 1205. In some cases, the I/O controller 1215may represent a physical connection or port to an external peripheral.In some cases, the I/O controller 1215 may utilize an operating systemsuch as iOS®, ANDROID®, MS-DOS®, MS-WINDOWS®, OS/2®, UNIX®, LINUX®, oranother known operating system. In other cases, the I/O controller 1215may represent or interact with a modem, a keyboard, a mouse, atouchscreen, or a similar device. In some cases, the I/O controller 1215may be implemented as part of a processor. In some cases, a user mayinteract with the device 1205 via the I/O controller 1215 or viahardware components controlled by the I/O controller 1215.

The transceiver 1220 may communicate bi-directionally, via one or moreantennas, wired, or wireless links as described above. For example, thetransceiver 1220 may represent a wireless transceiver and maycommunicate bi-directionally with another wireless transceiver. Thetransceiver 1220 may also include a modem to modulate the packets andprovide the modulated packets to the antennas for transmission, and todemodulate packets received from the antennas.

In some cases, the wireless device may include a single antenna 1225.However, in some cases the device may have more than one antenna 1225,which may be capable of concurrently transmitting or receiving multiplewireless transmissions.

The memory 1230 may include RAM and ROM. The memory 1230 may storecomputer-readable, computer-executable code 1235 including instructionsthat, when executed, cause the processor to perform various functionsdescribed herein. In some cases, the memory 1230 may contain, amongother things, a BIOS which may control basic hardware or softwareoperation such as the interaction with peripheral components or devices.

The processor 1240 may include an intelligent hardware device, (e.g., ageneral-purpose processor, a DSP, a CPU, a microcontroller, an ASIC, anFPGA, a programmable logic device, a discrete gate or transistor logiccomponent, a discrete hardware component, or any combination thereof).In some cases, the processor 1240 may be configured to operate a memoryarray using a memory controller. In other cases, a memory controller maybe integrated into the processor 1240. The processor 1240 may beconfigured to execute computer-readable instructions stored in a memory(e.g., the memory 1230) to cause the device 1205 to perform variousfunctions (e.g., functions or tasks supporting handover techniques inwireless communications).

The code 1235 may include instructions to implement aspects of thepresent disclosure, including instructions to support wirelesscommunications. The code 1235 may be stored in a non-transitorycomputer-readable medium such as system memory or other type of memory.In some cases, the code 1235 may not be directly executable by theprocessor 1240 but may cause a computer (e.g., when compiled andexecuted) to perform functions described herein.

FIG. 13 shows a block diagram 1300 of a device 1305 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The device 1305 may be an example ofaspects of a base station 105 as described herein. The device 1305 mayinclude a receiver 1310, a communications manager 1315, and atransmitter 1320. The device 1305 may also include a processor. Each ofthese components may be in communication with one another (e.g., via oneor more buses).

The receiver 1310 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to handovertechniques in wireless communications, etc.). Information may be passedon to other components of the device 1305. The receiver 1310 may be anexample of aspects of the transceiver 1620 described with reference toFIG. 16. The receiver 1310 may utilize a single antenna or a set ofantennas.

In some cases, the base station may be a source base station and thecommunications manager 1315 may establish a first connection with a UE,initiate a handover procedure to handover the UE to a second basestation, where the first connection with the first base station ismaintained during the handover procedure. In some cases, the basestation may discontinue the handover procedure to handover the UE to thesecond base station, based on an indication of a failure of the handoverprocedure.

In some cases, the base station may be a target base station and thecommunications manager 1315 may also receive a handover message toinitiate a handover of a UE from a first or source base station to thetarget base station, and initiate, responsive to the handover message, aconnection establishment with the UE to establish a second connectionbetween the UE and the target base station. In some cases, the targetbase station may receive from the UE an indication of a failure of afirst connection between the UE and the first base station, where thefirst connection was to be maintained during the handover of the UE fromthe first base station to the target base station, and forward theindication of the failure of the first connection to the first basestation.

In some cases, when the base station is a source base station, thecommunications manager 1315 may also establish a first connection with aUE, initiate a handover of the UE to a second base station, where thefirst connection with the first base station is maintained during thehandover and an establishment of a second connection between the UE andthe second base station, and may modify the handover of the UE based ona measurement report from the UE.

In some cases, when the base station is a target base station, thecommunications manager 1315 may also receive a handover message toinitiate a handover of a UE from the first base station to the targetbase station, modify the handover of the UE based on a measurementreport, establish a second connection with the UE responsive to thehandover message, and receive a measurement report from the UE thatincludes one or more channel measurements associated with the first basestation and the target base station.

In some cases, the base station may be a source base station, and mayreceive from a target base station a role switch indication thatindicates that the second connection between the UE and the target basestation is a primary connection of the UE. The communications manager1315 may also transmit, when the base station is a target base station,responsive to the establishing the second connection, a role switchindication to the source base station that indicates that the secondconnection between the UE and the target base station is a primaryconnection of the UE. The communications manager 1315 may be an exampleof aspects of the communications manager 1610 described herein.

The communications manager 1315, or its sub-components, may beimplemented in hardware, code (e.g., software or firmware) executed by aprocessor, or any combination thereof. If implemented in code executedby a processor, the functions of the communications manager 1315, or itssub-components may be executed by a general-purpose processor, a DSP, anapplication-specific integrated circuit (ASIC), a FPGA or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described in the present disclosure.

The communications manager 1315, or its sub-components, may bephysically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations by one or more physical components. In some examples, thecommunications manager 1315, or its sub-components, may be a separateand distinct component in accordance with various aspects of the presentdisclosure. In some examples, the communications manager 1315, or itssub-components, may be combined with one or more other hardwarecomponents, including but not limited to an input/output (I/O)component, a transceiver, a network server, another computing device,one or more other components described in the present disclosure, or acombination thereof in accordance with various aspects of the presentdisclosure.

The transmitter 1320 may transmit signals generated by other componentsof the device 1305. In some examples, the transmitter 1320 may becollocated with a receiver 1310 in a transceiver module. For example,the transmitter 1320 may be an example of aspects of the transceiver1620 described with reference to FIG. 16. The transmitter 1320 mayutilize a single antenna or a set of antennas.

FIG. 14 shows a block diagram 1400 of a device 1405 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The device 1405 may be an example ofaspects of a device 1305, or a base station 105 as described herein. Thedevice 1405 may include a receiver 1410, a communications manager 1415,and a transmitter 1445. The device 1405 may also include a processor.Each of these components may be in communication with one another (e.g.,via one or more buses).

The receiver 1410 may receive information such as packets, user data, orcontrol information associated with various information channels (e.g.,control channels, data channels, and information related to handovertechniques in wireless communications, etc.). Information may be passedon to other components of the device 1405. The receiver 1410 may be anexample of aspects of the transceiver 1620 described with reference toFIG. 16. The receiver 1410 may utilize a single antenna or a set ofantennas.

The communications manager 1415 may be an example of aspects of thecommunications manager 1315 as described herein. The communicationsmanager 1415 may include a connection establishment manager 1420, ahandover manager 1425, a RLF manager 1430, a random access manager 1435,and a measurement component 1440. The communications manager 1415 may bean example of aspects of the communications manager 1610 describedherein.

The connection establishment manager 1420 may establish, at a first basestation, a first connection with a UE.

The handover manager 1425 may initiate a handover procedure to handoverthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover procedure, anddiscontinue the handover procedure to handover the UE to the second basestation in some cases. In some cases, the handover manager 1425 maymodify the handover of the UE based on a measurement report. In somecases, the handover manager 1425 receive a role switch indication thatindicates a primary connection and secondary connection are to beswitched.

The RLF manager 1430 may receive, from the UE, an indication of afailure of the handover procedure. In some cases, the RLF manager 1430may be associated with a target second base station, and may receivefrom the UE an indication of a failure of a first connection between theUE and the first base station, where the first connection was to bemaintained during the handover of the UE from the first base station tothe second base station and forward the indication of the failure of thefirst connection to the first base station.

The random access manager 1435 may initiate, responsive to the handovermessage, a connection establishment with the UE to establish a secondconnection between the UE and the second base station.

The measurement component 1440 may receive a measurement reportresponsive to the initiating the handover. In some cases, themeasurement report from the UE may include one or more channelmeasurements associated with the first base station and the second basestation.

The transmitter 1445 may transmit signals generated by other componentsof the device 1405. In some examples, the transmitter 1445 may becollocated with a receiver 1410 in a transceiver module. For example,the transmitter 1445 may be an example of aspects of the transceiver1620 described with reference to FIG. 16. The transmitter 1445 mayutilize a single antenna or a set of antennas.

FIG. 15 shows a block diagram 1500 of a communications manager 1505 thatsupports handover techniques in wireless communications in accordancewith aspects of the present disclosure. The communications manager 1505may be an example of aspects of a communications manager 1315, acommunications manager 1415, or a communications manager 1610 describedherein. The communications manager 1505 may include a connectionestablishment manager 1510, a handover manager 1515, a RLF manager 1520,a measurement component 1525, a random access manager 1530, and a DCmanager 1535. Each of these modules may communicate, directly orindirectly, with one another (e.g., via one or more buses).

The connection establishment manager 1510 may establish, at a first basestation, a first connection with a UE. In some examples, the connectionestablishment manager 1510 may be associated with a second base stationand may establish a second connection with the UE responsive to thehandover message.

The handover manager 1515 may initiate a handover procedure to handoverthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover procedure. In someexamples, the handover manager 1515 may discontinue the handoverprocedure to handover the UE to the second base station. In someexamples, the handover manager 1515 may receive, at a second basestation, a handover message to initiate a handover of a UE from a firstbase station to the second base station.

In some examples, the handover manager 1515 may initiate a handover ofthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover and anestablishment of a second connection between the UE and the second basestation. In some examples, the handover manager 1515 may modify thehandover of the UE based on the measurement report.

In some examples, the handover manager 1515 may receive, from the secondbase station, a role switch indication that indicates that the secondconnection between the UE and the second base station is a primaryconnection of the UE. In some examples, the handover manager 1515 may beassociated with the second base station, and may transmit, responsive tothe establishing the second connection, a role switch indication to thefirst base station that indicates that the second connection between theUE and the second base station is a primary connection of the UE.

In some examples, the handover manager 1515 may be associated with afirst base station and may transmit, to the second base station andresponsive to the receiving the indication of the failure of thehandover procedure, a message to the second base station to cancel thehandover procedure. In some examples, the handover manager 1515 mayinitiate a second handover procedure to handover the UE to a third basestation, where the first connection with the first base station ismaintained during the second handover procedure. In some examples, thehandover manager 1515 may transmit, to the UE, a reconfiguration messageindicating the UE is to perform the second handover procedure with thethird base station.

In some examples, the handover manager 1515 may transmit an indicationto the second base station that the handover of the UE to the secondbase station is aborted. In some examples, the handover manager 1515 mayrelease the second connection after the first connection has been aprimary connection for a predetermined time period. In some examples,the handover manager 1515 may release the first connection with the UEresponsive to a role switch indication.

The RLF manager 1520 may receive, from the UE, an indication of afailure of the handover procedure. In some examples, the firstconnection was to be maintained during the handover of the UE from thefirst base station to the second base station. In some examples, the RLFmanager 1520 may forward the indication of the failure of the firstconnection to the first base station. In some examples, the radio linkfailure message from the UE may indicate a timer expiration associatedwith a random access procedure to establish a second connection with thesecond base station.

In some examples, the RLF manager 1520 may receive a radio link failuremessage from the UE indicating a second connection with the second basestation was established and then failed. In some cases, the radio linkfailure message indicates a failure cause associated with the secondconnection, and where the failure cause indicates one or more or a timerexpiration associated with the second connection, a random accessprocedure failure, or a maximum number of retransmissions is reached fora communication using the second connection. In some cases, the radiolink failure message further indicates a target cell groupidentification associated with the second base station. In some cases,the indication of the failure of the first connection indicates thatresources and context associated with the first connection are to bedeleted. In some cases, the indication of the failure of the firstconnection includes a failure cause associated with the first connectionand a source cell group identification associated with the first basestation.

The measurement component 1525 may receive a measurement reportresponsive to the initiating the handover. In some examples, themeasurement component 1525 may receive a measurement report from the UEthat includes one or more channel measurements associated with the firstbase station and the second base station. In some examples, themeasurement component 1525 may receive one or more additionalmeasurement reports.

In some cases, the indication of the failure of the handover procedurefurther indicates a measurement of one or more signals received at theUE from a neighboring base station. In some cases, the indication of thefailure of the first connection further includes a measurement of one ormore signals received at the UE from a neighboring base station. In somecases, the measurement report is transmitted responsive to the measuredone or more channel conditions of the secondary connection being betterthan corresponding channel conditions of the primary connection. In somecases, the measurement report is transmitted responsive to measuredchannel conditions of a third base station being better than measuredchannel conditions of the second base station.

The random access manager 1530 may initiate, responsive to the handovermessage, a connection establishment with the UE to establish a secondconnection between the UE and the second base station.

The DC manager 1535 may receive, from the second base station prior toreceiving the measurement report, a role switch indication thatindicates that a second connection between the UE and the second basestation is a primary connection of the UE and the first connection is asecondary connection. In some examples, the modifying the handover ofthe UE includes reconfiguring the first connection to be the primaryconnection and the second connection to be the secondary connectionbased on the measurement report. In some examples, the DC manager 1535may further reconfigure the primary connection and the secondaryconnection based on the one or more additional measurement reports.

In some examples, the DC manager 1535 may exchange information with thesecond base station to establish duplication of data communicated withthe UE via both the first connection and the second connection. In someexamples, the DC manager 1535 may communicate duplicated data with theUE via the first connection. In some examples, the DC manager 1535 maytransmit, responsive to the measurement report, a reconfigurationmessage to the UE to abort the handover procedure with the second basestation.

In some examples, the DC manager 1535 may transmit, to the first basestation prior to receiving the measurement report, a role switchindication that indicates that a second connection between the UE andthe second base station is a primary connection of the UE and a firstconnection between the UE and the first base station is a secondaryconnection. In some examples, the modifying the handover of the UEincludes reconfiguring the first connection to be the primary connectionand the second connection to be the secondary connection based on themeasurement report.

In some examples, the DC manager 1535 may configure one or moresecondary node (SN) terminated bearers at the second base station forthe second connection. In some examples, the DC manager 1535 maytransmit a reconfiguration message to the UE that indicates the handoverto the second base station. In some examples, the DC manager 1535 maytransmit a role switch request to the second base station. In someexamples, the DC manager 1535 may receive an acknowledgment of the roleswitch request from the second base station.

In some cases, the role switch request is transmitted with a SN additionrequest to the second base station, and the acknowledgment of the roleswitch request is received with a SN addition acknowledgment from thesecond base station. In some cases, the reconfiguration message to theUE indicates a secondary carrier group associated with the secondconnection and a source cell group identification associated with thefirst connection. In some cases, the role switch indication is receivedsubsequent to performance of a random access procedure between thesecond base station and the UE.

FIG. 16 shows a diagram of a system 1600 including a device 1605 thatsupports handover techniques in wireless communications in accordancewith aspects of the present disclosure. The device 1605 may be anexample of or include the components of device 1305, device 1405, or abase station 105 as described herein. The device 1605 may includecomponents for bi-directional voice and data communications includingcomponents for transmitting and receiving communications, including acommunications manager 1610, a network communications manager 1615, atransceiver 1620, an antenna 1625, memory 1630, a processor 1640, and aninter-station communications manager 1645. These components may be inelectronic communication via one or more buses (e.g., bus 1650).

The communications manager 1610 may establish, at a first base station,a first connection with a UE, initiate a handover procedure to handoverthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover procedure,discontinue the handover procedure to handover the UE to the second basestation, and receive, from the UE, an indication of a failure of thehandover procedure. The communications manager 1610 may also receive, ata second base station, a handover message to initiate a handover of a UEfrom a first base station to the second base station, initiate,responsive to the handover message, a connection establishment with theUE to establish a second connection between the UE and the second basestation, receive from the UE an indication of a failure of a firstconnection between the UE and the first base station, where the firstconnection was to be maintained during the handover of the UE from thefirst base station to the second base station, and forward theindication of the failure of the first connection to the first basestation. The communications manager 1610 may also establish, at a firstbase station, a first connection with a UE, initiate a handover of theUE to a second base station, where the first connection with the firstbase station is maintained during the handover and an establishment of asecond connection between the UE and the second base station, modify thehandover of the UE based on the measurement report, and receive ameasurement report responsive to the initiating the handover. Thecommunications manager 1610 may also receive, at a second base stationfrom a first base station, a handover message to initiate a handover ofa UE from the first base station to the second base station, modify thehandover of the UE based on the measurement report, establish a secondconnection with the UE responsive to the handover message, and receive ameasurement report from the UE that includes one or more channelmeasurements associated with the first base station and the second basestation. The communications manager 1610 may also establish, at a firstbase station, a first connection with a UE, initiate a handover of theUE to a second base station, where the first connection with the firstbase station is maintained during the handover and an establishment of asecond connection between the UE and the second base station, andreceive, from the second base station, a role switch indication thatindicates that the second connection between the UE and the second basestation is a primary connection of the UE. The communications manager1610 may also receive, at a second base station from a first basestation, a handover message to initiate a handover of a UE from thefirst base station to the second base station, transmit, responsive tothe establishing the second connection, a role switch indication to thefirst base station that indicates that the second connection between theUE and the second base station is a primary connection of the UE, andestablish a second connection with the UE responsive to the handovermessage.

The network communications manager 1615 may manage communications withthe core network (e.g., via one or more wired backhaul links). Forexample, the network communications manager 1615 may manage the transferof data communications for client devices, such as one or more UEs 115.

The transceiver 1620 may communicate bi-directionally, via one or moreantennas, wired, or wireless links as described above. For example, thetransceiver 1620 may represent a wireless transceiver and maycommunicate bi-directionally with another wireless transceiver. Thetransceiver 1620 may also include a modem to modulate the packets andprovide the modulated packets to the antennas for transmission, and todemodulate packets received from the antennas.

In some cases, the wireless device may include a single antenna 1625.However, in some cases the device may have more than one antenna 1625,which may be capable of concurrently transmitting or receiving multiplewireless transmissions.

The memory 1630 may include RAM, ROM, or a combination thereof. Thememory 1630 may store computer-readable code 1635 including instructionsthat, when executed by a processor (e.g., the processor 1640) cause thedevice to perform various functions described herein. In some cases, thememory 1630 may contain, among other things, a BIOS which may controlbasic hardware or software operation such as the interaction withperipheral components or devices.

The processor 1640 may include an intelligent hardware device, (e.g., ageneral-purpose processor, a DSP, a CPU, a microcontroller, an ASIC, anFPGA, a programmable logic device, a discrete gate or transistor logiccomponent, a discrete hardware component, or any combination thereof).In some cases, the processor 1640 may be configured to operate a memoryarray using a memory controller. In some cases, a memory controller maybe integrated into processor 1640. The processor 1640 may be configuredto execute computer-readable instructions stored in a memory (e.g., thememory 1630) to cause the device 1605 to perform various functions(e.g., functions or tasks supporting handover techniques in wirelesscommunications).

The inter-station communications manager 1645 may manage communicationswith other base station 105, and may include a controller or schedulerfor controlling communications with UEs 115 in cooperation with otherbase stations 105. For example, the inter-station communications manager1645 may coordinate scheduling for transmissions to UEs 115 for variousinterference mitigation techniques such as beamforming or jointtransmission. In some examples, the inter-station communications manager1645 may provide an X2 interface within an LTE/LTE-A wirelesscommunication network technology to provide communication between basestations 105.

The code 1635 may include instructions to implement aspects of thepresent disclosure, including instructions to support wirelesscommunications. The code 1635 may be stored in a non-transitorycomputer-readable medium such as system memory or other type of memory.In some cases, the code 1635 may not be directly executable by theprocessor 1640 but may cause a computer (e.g., when compiled andexecuted) to perform functions described herein.

FIG. 17 shows a flowchart illustrating a method 1700 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 1700 may beimplemented by a UE 115 or its components as described herein. Forexample, the operations of method 1700 may be performed by acommunications manager as described with reference to FIGS. 9 through12. In some examples, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 1705, the UE may establish a first connection with a first basestation. The operations of 1705 may be performed according to themethods described herein. In some examples, aspects of the operations of1705 may be performed by a connection establishment manager as describedwith reference to FIGS. 9 through 12.

At 1710, the UE may receive a handover message from the first basestation to perform a handover procedure with a second base station. Theoperations of 1710 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1710 may beperformed by a handover manager as described with reference to FIGS. 9through 12.

At 1715, the UE may transmit, responsive to the handover message, arequest to the second base station to establish a second connection withthe second base station, where the first connection with the first basestation is maintained during the handover procedure. The operations of1715 may be performed according to the methods described herein. In someexamples, aspects of the operations of 1715 may be performed by a randomaccess manager as described with reference to FIGS. 9 through 12.

At 1720, the UE may determine that a radio link failure of the firstconnection or the second connection has occurred. The operations of 1720may be performed according to the methods described herein. In someexamples, aspects of the operations of 1720 may be performed by a RLFmanager as described with reference to FIGS. 9 through 12.

At 1725, the UE may transmit an indication of the radio link failure tothe first base station or the second base station responsive to thedetermining. The operations of 1725 may be performed according to themethods described herein. In some examples, aspects of the operations of1725 may be performed by a RLF manager as described with reference toFIGS. 9 through 12.

FIG. 18 shows a flowchart illustrating a method 1800 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 1800 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 1800 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 1805, the base station may be a first base station (e.g., a sourcebase station), and may establish a first connection with a UE. Theoperations of 1805 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1805 may beperformed by a connection establishment manager as described withreference to FIGS. 13 through 16.

At 1810, the base station may initiate a handover procedure to handoverthe UE to a second base station, where the first connection with thefirst base station is maintained during the handover procedure. Theoperations of 1810 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1810 may beperformed by a handover manager as described with reference to FIGS. 13through 16.

At 1815, the base station may receive, from the UE, an indication of afailure of the handover procedure. The operations of 1815 may beperformed according to the methods described herein. In some examples,aspects of the operations of 1815 may be performed by a RLF manager asdescribed with reference to FIGS. 13 through 16.

At 1820, the base station may discontinue the handover procedure tohandover the UE to the second base station. The operations of 1820 maybe performed according to the methods described herein. In someexamples, aspects of the operations of 1820 may be performed by ahandover manager as described with reference to FIGS. 13 through 16.

FIG. 19 shows a flowchart illustrating a method 1900 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 1900 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 1900 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 1905, the base station may be a second base station (e.g., a targetbase station), and may receive a handover message to initiate a handoverof a UE from a first base station to the second base station. Theoperations of 1905 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1905 may beperformed by a handover manager as described with reference to FIGS. 13through 16.

At 1910, the base station may initiate, responsive to the handovermessage, a connection establishment with the UE to establish a secondconnection between the UE and the second base station. The operations of1910 may be performed according to the methods described herein. In someexamples, aspects of the operations of 1910 may be performed by a randomaccess manager as described with reference to FIGS. 13 through 16.

At 1915, the base station may receive from the UE an indication of afailure of a first connection between the UE and the first base station,where the first connection was to be maintained during the handover ofthe UE from the first base station to the second base station. Theoperations of 1915 may be performed according to the methods describedherein. In some examples, aspects of the operations of 1915 may beperformed by a RLF manager as described with reference to FIGS. 13through 16.

At 1920, the base station may forward the indication of the failure ofthe first connection to the first base station. The operations of 1920may be performed according to the methods described herein. In someexamples, aspects of the operations of 1920 may be performed by a RLFmanager as described with reference to FIGS. 13 through 16.

FIG. 20 shows a flowchart illustrating a method 2000 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2000 may beimplemented by a UE 115 or its components as described herein. Forexample, the operations of method 2000 may be performed by acommunications manager as described with reference to FIGS. 9 through12. In some examples, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2005, the UE may establish a first connection with a first basestation. The operations of 2005 may be performed according to themethods described herein. In some examples, aspects of the operations of2005 may be performed by a connection establishment manager as describedwith reference to FIGS. 9 through 12.

At 2010, the UE may receive a handover message from the first basestation to perform a handover procedure with a second base station. Theoperations of 2010 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2010 may beperformed by a handover manager as described with reference to FIGS. 9through 12.

At 2015, the UE may initiate a connection establishment with the secondbase station to establish a second connection responsive to the handovermessage, where the first connection is maintained during the connectionestablishment with the second base station. The operations of 2015 maybe performed according to the methods described herein. In someexamples, aspects of the operations of 2015 may be performed by a randomaccess manager as described with reference to FIGS. 9 through 12.

At 2020, the UE may measure one or more channel conditions associatedwith one or more of the first base station, the second base station or athird base station. The operations of 2020 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2020 may be performed by a measurement component asdescribed with reference to FIGS. 9 through 12.

At 2025, the UE may transmit, responsive to the handover message, ameasurement report to at least one of the first base station or thesecond base station responsive to measuring the one or more channelconditions. The operations of 2025 may be performed according to themethods described herein. In some examples, aspects of the operations of2025 may be performed by a measurement component as described withreference to FIGS. 9 through 12.

FIG. 21 shows a flowchart illustrating a method 2100 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2100 may beimplemented by a UE 115 or its components as described herein. Forexample, the operations of method 2100 may be performed by acommunications manager as described with reference to FIGS. 9 through12. In some examples, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2105, the UE may establish a first connection with a first basestation. The operations of 2105 may be performed according to themethods described herein. In some examples, aspects of the operations of2105 may be performed by a connection establishment manager as describedwith reference to FIGS. 9 through 12.

At 2110, the UE may receive a handover message from the first basestation to perform a handover procedure with a second base station. Theoperations of 2110 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2110 may beperformed by a handover manager as described with reference to FIGS. 9through 12.

At 2115, the UE may initiate a connection establishment with the secondbase station to establish a second connection responsive to the handovermessage, where the first connection is maintained during the connectionestablishment with the second base station. The operations of 2115 maybe performed according to the methods described herein. In someexamples, aspects of the operations of 2115 may be performed by a randomaccess manager as described with reference to FIGS. 9 through 12.

At 2120, the UE may measure one or more channel conditions associatedwith one or more of the first base station, the second base station or athird base station. The operations of 2120 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2120 may be performed by a measurement component asdescribed with reference to FIGS. 9 through 12.

At 2125, the UE may transmit, responsive to the handover message, ameasurement report to at least one of the first base station or thesecond base station responsive to measuring the one or more channelconditions. The operations of 2125 may be performed according to themethods described herein. In some examples, aspects of the operations of2125 may be performed by a measurement component as described withreference to FIGS. 9 through 12.

At 2130, the UE may establish the second connection with the second basestation, where the second connection is a primary connection and thefirst connection is maintained as a secondary connection. The operationsof 2130 may be performed according to the methods described herein. Insome examples, aspects of the operations of 2130 may be performed by aDC manager as described with reference to FIGS. 9 through 12.

At 2135, the UE may receive, responsive to the measurement report, areconfiguration message from the second base station to reconfigure thefirst connection as the primary connection and the second connection asthe secondary connection. The operations of 2135 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2135 may be performed by a DC manager as describedwith reference to FIGS. 9 through 12.

At 2140, the UE may reconfigure the first connection and the secondconnection responsive to the reconfiguration message. The operations of2140 may be performed according to the methods described herein. In someexamples, aspects of the operations of 2140 may be performed by a DCmanager as described with reference to FIGS. 9 through 12.

At 2145, the UE may transmit one or more additional measurement reportsvia the primary connection. The operations of 2145 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2145 may be performed by a measurement component asdescribed with reference to FIGS. 9 through 12.

FIG. 22 shows a flowchart illustrating a method 2200 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2200 may beimplemented by a UE 115 or its components as described herein. Forexample, the operations of method 2200 may be performed by acommunications manager as described with reference to FIGS. 9 through12. In some examples, a UE may execute a set of instructions to controlthe functional elements of the UE to perform the functions describedbelow. Additionally or alternatively, a UE may perform aspects of thefunctions described below using special-purpose hardware.

At 2205, the UE may establish a first connection with a first basestation. The operations of 2205 may be performed according to themethods described herein. In some examples, aspects of the operations of2205 may be performed by a connection establishment manager as describedwith reference to FIGS. 9 through 12.

At 2210, the UE may receive a handover message from the first basestation to perform a handover procedure with a second base station. Theoperations of 2210 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2210 may beperformed by a handover manager as described with reference to FIGS. 9through 12.

At 2215, the UE may initiate a connection establishment with the secondbase station to establish a second connection responsive to the handovermessage, where the first connection is maintained during the connectionestablishment with the second base station. The operations of 2215 maybe performed according to the methods described herein. In someexamples, aspects of the operations of 2215 may be performed by a randomaccess manager as described with reference to FIGS. 9 through 12.

At 2220, the UE may measure one or more channel conditions associatedwith one or more of the first base station, the second base station or athird base station. The operations of 2220 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2220 may be performed by a measurement component asdescribed with reference to FIGS. 9 through 12.

At 2225, the UE may transmit, responsive to the handover message, ameasurement report to at least one of the first base station or thesecond base station responsive to measuring the one or more channelconditions. The operations of 2225 may be performed according to themethods described herein. In some examples, aspects of the operations of2225 may be performed by a measurement component as described withreference to FIGS. 9 through 12.

At 2230, the UE may receive, responsive to the measurement report, areconfiguration message from the first base station to abort thehandover procedure with the second base station. The operations of 2230may be performed according to the methods described herein. In someexamples, aspects of the operations of 2230 may be performed by a DCmanager as described with reference to FIGS. 9 through 12. In somecases, the measurement report is transmitted responsive to measuredchannel conditions of the third base station being better than measuredchannel conditions of the second base station. In some cases, thereconfiguration message further includes a second handover message fromthe first base station to perform a second handover procedure with thethird base station.

FIG. 23 shows a flowchart illustrating a method 2300 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2300 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2300 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2305, the base station may be a first base station (e.g., a sourcebase station), and may establish a first connection with a UE. Theoperations of 2305 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2305 may beperformed by a connection establishment manager as described withreference to FIGS. 13 through 16.

At 2310, the base station may initiate a handover of the UE to a secondbase station, where the first connection with the first base station ismaintained during the handover and an establishment of a secondconnection between the UE and the second base station. The operations of2310 may be performed according to the methods described herein. In someexamples, aspects of the operations of 2310 may be performed by ahandover manager as described with reference to FIGS. 13 through 16.

At 2315, the base station may receive a measurement report responsive tothe initiating the handover. The operations of 2315 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2315 may be performed by a measurement component asdescribed with reference to FIGS. 13 through 16.

At 2320, the base station may modify the handover of the UE based on themeasurement report. The operations of 2320 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2320 may be performed by a handover manager as describedwith reference to FIGS. 13 through 16.

FIG. 24 shows a flowchart illustrating a method 2400 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2400 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2400 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2405, the base station may be a first base station (e.g., a sourcebase station), and may establish a first connection with a UE. Theoperations of 2405 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2405 may beperformed by a connection establishment manager as described withreference to FIGS. 13 through 16.

At 2410, the base station may initiate a handover of the UE to a secondbase station, where the first connection with the first base station ismaintained during the handover and an establishment of a secondconnection between the UE and the second base station. The operations of2410 may be performed according to the methods described herein. In someexamples, aspects of the operations of 2410 may be performed by ahandover manager as described with reference to FIGS. 13 through 16.

At 2415, the base station may receive, from the second base station, arole switch indication that indicates that a second connection betweenthe UE and the second base station is a primary connection of the UE andthe first connection is a secondary connection. The operations of 2415may be performed according to the methods described herein. In someexamples, aspects of the operations of 2415 may be performed by a DCmanager as described with reference to FIGS. 13 through 16.

At 2420, the base station may receive a measurement report responsive tothe initiating the handover. The operations of 2420 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2420 may be performed by a measurement component asdescribed with reference to FIGS. 13 through 16.

At 2425, the base station may modify the handover of the UE based on themeasurement report. The operations of 2425 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2425 may be performed by a handover manager as describedwith reference to FIGS. 13 through 16. In some cases, the modifying thehandover of the UE includes reconfiguring the first connection to be theprimary connection and the second connection to be the secondaryconnection based on the measurement report.

At 2430, the base station may receive one or more additional measurementreports. The operations of 2430 may be performed according to themethods described herein. In some examples, aspects of the operations of2430 may be performed by a measurement component as described withreference to FIGS. 13 through 16.

At 2435, the base station may further reconfigure the primary connectionand the secondary connection based on the one or more additionalmeasurement reports. The operations of 2445 may be performed accordingto the methods described herein. In some examples, aspects of theoperations of 2445 may be performed by a DC manager as described withreference to FIGS. 13 through 16. In some cases, the measurement reportis transmitted responsive to the measured one or more channel conditionsof the secondary connection being better than corresponding channelconditions of the primary connection.

FIG. 25 shows a flowchart illustrating a method 2500 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2500 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2500 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2505, the base station may be a second base station (e.g., a targetbase station), and may receive, from a first base station, a handovermessage to initiate a handover of a UE from the first base station tothe second base station. The operations of 2505 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2505 may be performed by a handover manager asdescribed with reference to FIGS. 13 through 16.

At 2510, the base station may establish a second connection with the UEresponsive to the handover message. The operations of 2510 may beperformed according to the methods described herein. In some examples,aspects of the operations of 2510 may be performed by a connectionestablishment manager as described with reference to FIGS. 13 through16.

At 2515, the base station may receive a measurement report from the UEthat includes one or more channel measurements associated with the firstbase station and the second base station. The operations of 2515 may beperformed according to the methods described herein. In some examples,aspects of the operations of 2515 may be performed by a measurementcomponent as described with reference to FIGS. 13 through 16.

At 2520, the base station may modify the handover of the UE based on themeasurement report. The operations of 2520 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2520 may be performed by a handover manager as describedwith reference to FIGS. 13 through 16.

FIG. 26 shows a flowchart illustrating a method 2600 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2600 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2600 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2605, the base station may be a second base station (e.g., a targetbase station), and may receive, from a first base station, a handovermessage to initiate a handover of a UE from the first base station tothe second base station. The operations of 2605 may be performedaccording to the methods described herein. In some examples, aspects ofthe operations of 2605 may be performed by a handover manager asdescribed with reference to FIGS. 13 through 16.

At 2610, the base station may establish a second connection with the UEresponsive to the handover message. The operations of 2610 may beperformed according to the methods described herein. In some examples,aspects of the operations of 2610 may be performed by a connectionestablishment manager as described with reference to FIGS. 13 through16.

At 2615, the base station may transmit, to the first base station, arole switch indication that indicates that a second connection betweenthe UE and the second base station is a primary connection of the UE anda first connection between the UE and the first base station is asecondary connection. The operations of 2615 may be performed accordingto the methods described herein. In some examples, aspects of theoperations of 2615 may be performed by a DC manager as described withreference to FIGS. 13 through 16.

At 2620, the base station may receive a measurement report from the UEthat includes one or more channel measurements associated with the firstbase station and the second base station. The operations of 2620 may beperformed according to the methods described herein. In some examples,aspects of the operations of 2620 may be performed by a measurementcomponent as described with reference to FIGS. 13 through 16.

At 2625, the base station may modify the handover of the UE based on themeasurement report. The operations of 2625 may be performed according tothe methods described herein. In some examples, aspects of theoperations of 2625 may be performed by a handover manager as describedwith reference to FIGS. 13 through 16. In some cases, the modifying thehandover of the UE includes reconfiguring the first connection to be theprimary connection and the second connection to be the secondaryconnection based on the measurement report.

At 2630, the base station may receive one or more additional measurementreports. The operations of 2630 may be performed according to themethods described herein. In some examples, aspects of the operations of2630 may be performed by a measurement component as described withreference to FIGS. 13 through 16.

At 2635, the base station may further reconfigure the primary connectionand the secondary connection based on the one or more additionalmeasurement reports. The operations of 2635 may be performed accordingto the methods described herein. In some examples, aspects of theoperations of 2635 may be performed by a DC manager as described withreference to FIGS. 13 through 16.

FIG. 27 shows a flowchart illustrating a method 2700 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2700 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2700 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2705, the base station may establish a first connection with a UE.The operations of 2705 may be performed according to the methodsdescribed herein. In some examples, aspects of the operations of 2705may be performed by a connection establishment manager as described withreference to FIGS. 13 through 16.

At 2710, the base station may initiate a handover of the UE to a secondbase station, where the first connection with the first base station ismaintained during the handover and an establishment of a secondconnection between the UE and the second base station. The operations of2710 may be performed according to the methods described herein. In someexamples, aspects of the operations of 2710 may be performed by ahandover manager as described with reference to FIGS. 13 through 16.

At 2715, the base station may receive, from the second base station, arole switch indication that indicates that the second connection betweenthe UE and the second base station is a primary connection of the UE.The operations of 2715 may be performed according to the methodsdescribed herein. In some examples, aspects of the operations of 2715may be performed by a handover manager as described with reference toFIGS. 13 through 16.

FIG. 28 shows a flowchart illustrating a method 2800 that supportshandover techniques in wireless communications in accordance withaspects of the present disclosure. The operations of method 2800 may beimplemented by a base station 105 or its components as described herein.For example, the operations of method 2800 may be performed by acommunications manager as described with reference to FIGS. 13 through16. In some examples, a base station may execute a set of instructionsto control the functional elements of the base station to perform thefunctions described below. Additionally or alternatively, a base stationmay perform aspects of the functions described below usingspecial-purpose hardware.

At 2805, the base station may be a second base station and may receive,from a first base station, a handover message to initiate a handover ofa UE from the first base station to the second base station. Theoperations of 2805 may be performed according to the methods describedherein. In some examples, aspects of the operations of 2805 may beperformed by a handover manager as described with reference to FIGS. 13through 16.

At 2810, the base station may establish a second connection with the UEresponsive to the handover message. The operations of 2810 may beperformed according to the methods described herein. In some examples,aspects of the operations of 2810 may be performed by a random accessmanager as described with reference to FIGS. 13 through 16.

At 2815, the base station may transmit, responsive to the establishingthe second connection, a role switch indication to the first basestation that indicates that the second connection between the UE and thesecond base station is a primary connection of the UE. The operations of2815 may be performed according to the methods described herein. In someexamples, aspects of the operations of 2815 may be performed by ahandover manager as described with reference to FIGS. 13 through 16.

It should be noted that the methods described herein describe possibleimplementations, and that the operations and the steps may be rearrangedor otherwise modified and that other implementations are possible.Further, aspects from two or more of the methods may be combined.

Techniques described herein may be used for various wirelesscommunications systems such as code division multiple access (CDMA),time division multiple access (TDMA), frequency division multiple access(FDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and other systems.A CDMA system may implement a radio technology such as CDMA2000,Universal Terrestrial Radio Access (UTRA), etc. CDMA2000 covers IS-2000,IS-95, and IS-856 standards. IS-2000 Releases may be commonly referredto as CDMA2000 1×, 1×, etc. IS-856 (TIA-856) is commonly referred to asCDMA2000 1×EV-DO, High Rate Packet Data (HRPD), etc. UTRA includesWideband CDMA (WCDMA) and other variants of CDMA. A TDMA system mayimplement a radio technology such as Global System for MobileCommunications (GSM).

An OFDMA system may implement a radio technology such as Ultra MobileBroadband (UMB), Evolved UTRA (E-UTRA), Institute of Electrical andElectronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE802.20, Flash-OFDM, etc. UTRA and E-UTRA are part of Universal MobileTelecommunications System (UMTS). LTE, LTE-A, and LTE-A Pro are releasesof UMTS that use E-UTRA. UTRA, E-UTRA, UMTS, LTE, LTE-A, LTE-A Pro, NR,and GSM are described in documents from the organization named “3rdGeneration Partnership Project” (3GPP). CDMA2000 and UMB are describedin documents from an organization named “3rd Generation PartnershipProject 2” (3GPP2). The techniques described herein may be used for thesystems and radio technologies mentioned herein as well as other systemsand radio technologies. While aspects of an LTE, LTE-A, LTE-A Pro, or NRsystem may be described for purposes of example, and LTE, LTE-A, LTE-APro, or NR terminology may be used in much of the description, thetechniques described herein are applicable beyond LTE, LTE-A, LTE-A Pro,or NR applications.

A macro cell generally covers a relatively large geographic area (e.g.,several kilometers in radius) and may allow unrestricted access by UEswith service subscriptions with the network provider. A small cell maybe associated with a lower-powered base station, as compared with amacro cell, and a small cell may operate in the same or different (e.g.,licensed, unlicensed, etc.) frequency bands as macro cells. Small cellsmay include pico cells, femto cells, and micro cells according tovarious examples. A pico cell, for example, may cover a small geographicarea and may allow unrestricted access by UEs with service subscriptionswith the network provider. A femto cell may also cover a smallgeographic area (e.g., a home) and may provide restricted access by UEshaving an association with the femto cell (e.g., UEs in a closedsubscriber group (CSG), UEs for users in the home, and the like). An eNBfor a macro cell may be referred to as a macro eNB. An eNB for a smallcell may be referred to as a small cell eNB, a pico eNB, a femto eNB, ora home eNB. An eNB may support one or multiple (e.g., two, three, four,and the like) cells, and may also support communications using one ormultiple component carriers.

The wireless communications systems described herein may supportsynchronous or asynchronous operation. For synchronous operation, thebase stations may have similar frame timing, and transmissions fromdifferent base stations may be approximately aligned in time. Forasynchronous operation, the base stations may have different frametiming, and transmissions from different base stations may not bealigned in time. The techniques described herein may be used for eithersynchronous or asynchronous operations.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection withthe disclosure herein may be implemented or performed with ageneral-purpose processor, a DSP, an ASIC, an FPGA, or otherprogrammable logic device, discrete gate or transistor logic, discretehardware components, or any combination thereof designed to perform thefunctions described herein. A general-purpose processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices(e.g., a combination of a DSP and a microprocessor, multiplemicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration).

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on acomputer-readable medium. Other examples and implementations are withinthe scope of the disclosure and appended claims. For example, due to thenature of software, functions described herein can be implemented usingsoftware executed by a processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations.

Computer-readable media includes both non-transitory computer storagemedia and communication media including any medium that facilitatestransfer of a computer program from one place to another. Anon-transitory storage medium may be any available medium that can beaccessed by a general purpose or special purpose computer. By way ofexample, and not limitation, non-transitory computer-readable media mayinclude random-access memory (RAM), read-only memory (ROM), electricallyerasable programmable ROM (EEPROM), flash memory, compact disk (CD) ROMor other optical disk storage, magnetic disk storage or other magneticstorage devices, or any other non-transitory medium that can be used tocarry or store desired program code means in the form of instructions ordata structures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, include CD, laser disc, optical disc,digital versatile disc (DVD), floppy disk and Blu-ray disc where disksusually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above are also includedwithin the scope of computer-readable media.

As used herein, including in the claims, “or” as used in a list of items(e.g., a list of items prefaced by a phrase such as “at least one of” or“one or more of”) indicates an inclusive list such that, for example, alist of at least one of A, B, or C means A or B or C or AB or AC or BCor ABC (i.e., A and B and C). Also, as used herein, the phrase “basedon” shall not be construed as a reference to a closed set of conditions.For example, an exemplary step that is described as “based on conditionA” may be based on both a condition A and a condition B withoutdeparting from the scope of the present disclosure. In other words, asused herein, the phrase “based on” shall be construed in the same manneras the phrase “based at least in part on.”

In the appended figures, similar components or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If just the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label, or othersubsequent reference label.

The description set forth herein, in connection with the appendeddrawings, describes example configurations and does not represent allthe examples that may be implemented or that are within the scope of theclaims. The term “exemplary” used herein means “serving as an example,instance, or illustration,” and not “preferred” or “advantageous overother examples.” The detailed description includes specific details forthe purpose of providing an understanding of the described techniques.These techniques, however, may be practiced without these specificdetails. In some instances, well-known structures and devices are shownin block diagram form in order to avoid obscuring the concepts of thedescribed examples.

The description herein is provided to enable a person skilled in the artto make or use the disclosure. Various modifications to the disclosurewill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other variations withoutdeparting from the scope of the disclosure. Thus, the disclosure is notlimited to the examples and designs described herein, but is to beaccorded the broadest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method for wireless communication, comprising:establishing, at a user equipment (UE), a first connection with a firstbase station; receiving a handover message from the first base stationto perform a handover procedure with a second base station;transmitting, responsive to the handover message, a request to thesecond base station to establish a second connection with the secondbase station, wherein data communications via the first connection withthe first base station are maintained during the handover procedure;determining that a radio link failure of the first connection with thefirst base station has occurred prior to completion of an establishmentof the second connection; completing the establishment of the secondconnection with the second base station based at least in part ondetermining the radio link failure of the first connection; andtransmitting, via the second connection, an indication of the radio linkfailure to the second base station responsive to the determining,wherein the indication of the radio link failure includes a failurecause associated with the first connection and an identificationassociated with the first base station.
 2. The method of claim 1,further comprising: receiving a second handover message from the secondbase station to perform a second handover procedure with a third basestation, wherein data communications via the second connection aremaintained during the second handover procedure; determining thatestablishment of a third connection with the third base station hasfailed; and maintaining the second connection with the second basestation based at least in part on determining that the establishment ofthe third connection with the third base station has failed.
 3. Themethod of claim 1, wherein the indication of the radio link failurefurther indicates one or more of a timer expiration associated with thefirst connection, a random access procedure failure, or a maximum numberof retransmissions is reached for a communication using the firstconnection.
 4. The method of claim 1, wherein the indication of theradio link failure further indicates a measurement of one or moresignals received at the UE from one or more neighboring base stations.5. The method of claim 1, wherein the handover message comprises a radioresource control reconfiguration message.
 6. The method of claim 1,wherein the completing the establishment of the second connectionpreempts, based at least in part on determining the radio link failureof the first connection, a triggering of a reestablishment of the firstconnection with the first base station.
 7. The method of claim 1,further comprising: releasing the first connection and a source stackassociated with the first connection responsive to completing theestablishment of the second connection with the second base station. 8.The method of claim 1, wherein the indication of the radio link failureindicates a failure cause associated with the first connection and asource cell group identification associated with the first base station.9. A method for wireless communication, comprising: establishing, at afirst base station, a first connection with a user equipment (UE);initiating a handover procedure to handover the UE to a second basestation, wherein data communications via the first connection with thefirst base station are maintained during the handover procedure;receiving, from the UE, an indication of a failure of the handoverprocedure; transmitting, from the first base station to the second basestation and responsive to the receiving the indication of the failure ofthe handover procedure, a message to the second base station to cancelthe handover procedure; and discontinuing the handover procedure tohandover the UE to the second base station.
 10. The method of claim 9,wherein the receiving the indication of the failure of the handoverprocedure further comprises: receiving a radio link failure message fromthe UE indicating a timer expiration associated with a random accessprocedure to establish a second connection with the second base station.11. The method of claim 9, further comprising: receiving a radio linkfailure message from the UE indicating a second connection with thesecond base station was established and then failed.
 12. The method ofclaim 11, wherein the radio link failure message indicates a failurecause associated with the second connection, and wherein the failurecause indicates one or more or a timer expiration associated with thesecond connection, a random access procedure failure, or a maximumnumber of retransmissions is reached for a communication using thesecond connection.
 13. The method of claim 12, wherein the radio linkfailure message further indicates a target cell group identificationassociated with the second base station.
 14. The method of claim 9,wherein the indication of the failure of the handover procedure furtherindicates a measurement of one or more signals received at the UE from aneighboring base station.
 15. The method of claim 9, wherein initiatingthe handover procedure to handover the UE to the second base stationcomprises: transmitting a radio resource control reconfiguration messageto the UE comprising an indication to perform the handover procedurewith the second base station.
 16. The method of claim 9, furthercomprising: initiating a second handover procedure to handover the UE toa third base station, wherein data communications via the firstconnection with the first base station are maintained during the secondhandover procedure.
 17. The method of claim 16, wherein the initiatingthe second handover procedure further comprises: transmitting, to theUE, a reconfiguration message indicating the UE is to perform the secondhandover procedure with the third base station.
 18. A method forwireless communication, comprising: receiving, at a second base station,a handover message to initiate a handover of a user equipment (UE) froma first base station to the second base station; initiating, responsiveto the handover message, a connection establishment with the UE toestablish a second connection between the UE and the second basestation; performing a random access procedure with the UE to establishthe second connection between the UE and the second base station;receiving, via the second connection, from the UE an indication of afailure of a first connection between the UE and the first base station,wherein data communications during the handover are via the firstconnection prior to the failure of the first connection, and wherein theindication of the failure of the first connection is received subsequentto the establishment of the second connection, and wherein theindication of the failure of the first connection includes a failurecause associated with the first connection and an identificationassociated with the first base station; and forwarding the indication ofthe failure of the first connection to the first base.
 19. The method ofclaim 18, wherein the indication of the failure of the first connectionindicates that resources and context associated with the firstconnection are to be deleted.
 20. The method of claim 18, wherein thehandover message comprises a radio resource control reconfigurationmessage.
 21. The method of claim 18, wherein the indication of thefailure of the first connection further includes a measurement of one ormore signals received at the UE from a neighboring base station.
 22. Anapparatus for wireless communication, comprising: a processor, memory inelectronic communication with the processor; and instructions stored inthe memory and executable by the processor to cause the apparatus to:establish, at a user equipment (UE), a first connection with a firstbase station; receive a handover message from the first base station toperform a handover procedure with a second base station; transmit,responsive to the handover message, a request to the second base stationto establish a second connection with the second base station, whereindata communications via the first connection with the first base stationare maintained during the handover procedure; determine that a radiolink failure of the first connection with the first base station hasoccurred prior to completion of an establishment of the secondconnection; complete the establishment of the second connection with thesecond base station based at least in part on determining the radio linkfailure of the first connection; and transmit, via the secondconnection, an indication of the radio link failure to the second basestation responsive to the determining, wherein the indication of theradio link failure includes a failure cause associated with the firstconnection and an identification associated with the first base station.23. The apparatus of claim 22, wherein the instructions are furtherexecutable by the processor to cause the apparatus to: receive a secondhandover message from the second base station to perform a secondhandover procedure with a third base station, wherein datacommunications via the second connection are maintained during thesecond handover procedure; determine that establishment of a thirdconnection with the third base station has failed; and maintain thesecond connection with the second base station based at least in part ondetermining that the establishment of the third connection with thethird base station has failed.
 24. The apparatus of claim 22, whereinthe indication of the radio link failure further indicates one or moreof a timer expiration associated with the first connection, a randomaccess procedure failure, or a maximum number of retransmissions isreached for a communication using the first connection.
 25. Theapparatus of claim 22, wherein the indication of the radio link failurefurther indicates a measurement of one or more signals received at theUE from one or more neighboring base stations.
 26. The apparatus ofclaim 22, wherein the handover message comprises a radio resourcecontrol reconfiguration message.
 27. The apparatus of claim 22, whereinthe completing the establishment of the second connection preempts,based at least in part on determining the radio link failure of thefirst connection, a triggering of a reestablishment of the firstconnection with the first base station.